•-    •  ,i -•  !i*5v-;.- 

•Hi 


FOUNDATIONS   OF 
CHEMISTRY 


BY 

ARTHUR  A.  BLANCHARD,  PH.D. 

ASSOCIATE  PROFESSOR  OF  INORGANIC  CHEMISTRY  AT  THE 
MASSACHUSETTS  INSTITUTE  OF  TECHNOLOGY 


AND 


FRANK  B.  WADE,  B.S. 

HEAD  OF  THE  DEPARTMENT  OF  CHEMISTRY  AT  THE 
8HORTRIDGE  HIGH  SCHOOL,  INDIANAPOLIS,  INDIANA 


AMERICAN  BOOK  COMPANY 

NEW  YORK  CINCINNATI  CHICAGO 


COPYKIGHT,   1914, 

BY  A.  A.  BLANCHARD 
AND  FRANK  B.  WADE. 


COPYRIGHT,  1914,  IN  GBEVT  BRITAIN. 


BLANCHARD   AND   WADE,    FOUND.    OF  CHBM. 
W.  P.  I 


PREFACE 

FORMERLY,  the  study  of  the  classics  occupied  a  very 
prominent  position  in  educational  training,  and  many  of 
the  ablest  men  of  our  times  owe  their  efficiency  largely 
to  the  excellence  of  the  mental  discipline  thus  acquired. 
The  essential  mental  training  can,  however,  be  furnished 
in  the  study  of  subjects  that  possess  vital  interest  and 
present-day  usefulness,  provided  these  subjects  are  as 
well  taught  as  were  the  classics. 

A  tendency  is  now  evident  in  secondary  school  educa- 
tion to  depart  from  the  study  of  the  classics  and  to  substi- 
tute therefor  a  multitude  of  vocational  and  informational 
subjects.  It  is  extremely  likely  that  this  type  of  study 
will  not  only  fail  to  prove  as  practically  useful  as  its 
advocates  hope,  but  that  it  will  fail  to  impart  that  sturdy 
independence  of  thinking  —  that  ability  to  apply  what 
has  previously  been  gained  —  which  is  so  essential  to 
success  in  all  walks  of  life,  and  which  can  be  imparted 
in  large  measure  by  a  thorough  study  of  the  underlying 
principles  of  science  in  their  applications  to  well-selected 
cases. 

With  the  ideal  in  mind  of  teaching  the  scientific  method 
of  thought  while  considering  the  facts  and  principles  of 
chemistry,  the  authors  have  striven  to  write  a  book  the 
intelligent  study  of  which  will  develop  both  the  power 
of  the  pupil  to  think  originally  and  his  appreciation  of 
the  relation  between  the  subject  matter  of  chemistry  and 
his  daily  life. 

In  illustrating  the  principles  of  chemistry,  very  many 
important  industrial  processes  and  applications  to  daily 

3 

302077 


4  PREFACE 

lilV  have  beea  chosen,  but  the  greatest  effort  has  been 
made  to  keep  the  idea  uppermost  that  the  principles  con- 
cerned are  of  universal  application,  whereas  the  individual 
processes  are  transient  and  of  relatively  less  importance 
to  the  general  student  of  chemistry. 

In  the  development  of  the  subject,  the  plan  has  been 
to  proceed  from  simple  and  well-known  facts  to  others 
less  familiar.  The  consideration  of  chemical  theory  is 
deferred  to  later  chapters  so  that  a  good  basis  of  fact 
may  have  accumulated  before  the  theory  arising  from  the 
facts  is  considered. 

The  problems  at  the  end  of  the  chapters  are  graded  in 
difficulty  and  some  are  designed  to  tax  the  thinking  powers 
of  the  brightest  students,  for  it  is  true  that  a  course  in 
science  to  be  of  greatest  value  must  inspire  the  pupil  to 
think.  The  instructor  may  find  it  wise  in  some  cases  to 
select  from  the  list  of  questions  those  which  best  meet  the 
needs  of  his  own  students. 

In  the  opinion  of  the  authors,  the  first  twenty-four 
chapters  include  as  much  ground  as  can  be  thoroughly 
covered  by  the  average  high  school  pupil  in  a  year.  The 
subject  matter  of  the  remaining  chapters  is,  on  the  whole, 
more  difficult,  and  a  more  advanced  method  of  treatment 
has  been  used  in  order  that  more  mature  classes  may  con- 
tinue to  receive  mental  stimulus  from  the  work. 

In  conclusion,  the  authors  find  it  a  pleasant  duty  to 
acknowledge  encouragement  and  assistance  received  from 
many  of  their  friends. 

The  chapters  relating  to  food  chemistry  have  been  read 
by  Professor  A.  G.  Woodman  and  the  chapters  on  the 
carbon  compounds  by  Professor  F.  J.  Moore,  both  of  the 
Massachusetts  Institute  of  Technology,  and  both  of  these 
gentlemen  have  offered  many  helpful  suggestions.  The 


PREFACE  5 

chapter  on  Metallurgy  has  been  written  in  collaboration 
with  Professor  Carle  R.  Hayward,  of  the  Department 
of  Mining  and  Metallurgy  of  the  Massachusetts  Institute 
of  Technology.  The  entire  manuscript  has  been  read  by 
Professors  J.  W.  Phelan  and  Edward  Mueller,  of  the 
Massachusetts  Institute  of  Technology. 

Mr.  Charles  J.  Pieper,  of  the  University  High  School, 
Chicago,  Miss  Ellinor  Garber,  Mr.  Mont  K.  Baird,  and 
Mr.  E.  Vernon  Hahn,  of  the  Shortridge  High  School, 
Indianapolis,  have  made  many  valuable  suggestions.  Pho- 
tographs for  illustrations  have  been  furnished  by :  The 
Solvay  Process  Company,  W.  B.  Scaife  and  Sons  Com- 
pany, The  Carborundum  Company,  The  Goldschmidt 
Thermit  Company,  the  Loomis-Pettibone  Company,  and 
The  American  Museum  of  Natural  History. 


TABLE  OF  CONTENTS 


CHAPTER  PAGE 

I.  CHEMICAL  AND  PHYSICAL  CHANGES   ....        9 

II.  MIXTURES  AND  PURE  CHEMICAL  SUBSTANCES  .        .      15 

III.  ELEMENTS  AND  COMPOUNDS         .        .        .        .        .20 

IV.  COMBUSTION '  "*       '*'.'•        •      25 

V.    OXYGEN .38 

VI.  THE  OXIDES  OF  CARBON      .        .        ..       .        .        .50 

VII.  THE  ATMOSPHERE  AND  NITROGEN      ....      67 

VIII.  THE  GAS  LAWS     ........      79 

IX.     WATER .        .91 

X.    THE  COMPOSITION  OF  WATER 100 

XL  HYDROGEN     .        .        .        .        .        ...        .111 

XII.  THE  ATOMIC  THEORY  .        .        .        .        .        .        .121 

XIII.  HYDROGEN  CHLORIDE .    129 

XIV.  AVOGADRO'S  THEORY   . 141 

XV.  ATOMIC  AND  MOLECULAR  WEIGHTS;  SYMBOLS  AND 

FORMULAS 147 

XVI.  CHLORINE       .        .        ...        .        .        .        .    164 

XVII.  SODIUM  .        .        .        .     "  .        .        .        .        .        .177 

XVIII.  CALCIUM        .        .        .        .        .        .        .        .        .192 

XIX.  ACIDS,  BASES,  AND  SALTS;  NEUTRALIZATION   .        .    207 

XX.  NOMENCLATURE  OF  ACIDS,  BASES,  AND  SALTS          .     220 

XXL  METALS  .        ...        ...        .        .        .        .    225 

XXII.  METALLURGY         T       .        .        .        .        .        .        .239 

XXIII.  COMPOUNDS  OF  CARBON 264 

XXIV.  COMPOUNDS  OF  CARBON  (continued)   ....    282 

6 


TABLE  OF  CONTENTS 


CHAPTER 

XXV. 
XXVI. 
XXVII. 

THE  IONIC  THEORY  
ELECTROLYSIS     
ELECTROMOTIVE  SERIES    .        .        .        .        . 

PAGE 

.    299 
.    311 
327 

XXVIII. 
XXIX. 
XXX. 
XXXI. 
XXXII. 

HYDROGEN  EQUIVALENTS  AND  VALENCE 
SULPHUR  AND  ITS  COMPOUNDS 
NITROGEN  AND  ITS  COMPOUNDS 
THE  HALOGENS;  THE  PERIODIC  SYSTEM 
REVERSIBLE  REACTIONS  AND  EQUILIBRIUM    . 

.    333 
.    342 
.     358 
.    377 
.    392 

MATIONS 413 

APPENDIX : 

TABLE  OF  PRESSURE  OF  WATER  VAPOR            .        .        .  425 
METHOD  OF  CORRECTING  GAS  VOLUMES  FOR  THE  PRES- 
SURE OF  WATER  VAPOR 425 

SOME  PROPERTIES  OF  COMMON  ELEMENTS         .        .        .  427 

DENSITIES  OF  GASES 428 

SOME  PROPERTIES  OF  COMPOUNDS 429 

SOLUBILITY,  TABLE  AND  RULES 430 

IMPORTANT  TEMPERATURES 431 

COMPOSITION  OF  ALLOYS 432 

COMPOSITION  OF  THE  EARTH'S  CRUST        ....  433 

LOGARITHMS 434,  435 

METRIC  SYSTEM  OF  WEIGHTS  AND  MEASURES          .        .  436 

METRIC  EQUIVALENTS 436 

PERIODIC  ARRANGEMENT  OF  THE  ELEMENTS.     Facing  in- 
side back  cover. 

INTERNATIONAL  ATOMIC  WEIGHT  TABLE  FOR  1914.    In- 
side back  cover. 

INDEX                                                                                           ,  437 


FOUNDATIONS  OF  CHEMISTRY 

CHAPTER  I 

CHEMICAL   AND   PHYSICAL    CHANGES 

CHEMISTRY  is  the  science  which  deals  with  the  substances 
which  go  to  make  up  our  earth  and  everything  in  the  earth, 
—  sea,  atmosphere,  rocks,  plants,  and  animals.  Chemistry 
is  little  interested  in  the  size  or  shape  of  bodies  of  matter, 
its  concern  is  with  the  substances  of  which  the  bodies  are 
composed.  For  example,  a  large  irregular  ledge  of  marble 
is  marble,  and  is  to  the  chemist  the  same  substance  as  the 
marble  of  the  most  beautifully  chiseled  statue.  To  a  sculptor 
or  an  architect  the  size  and  form  of  a  marble  block  are  of 
supreme  importance ;  but  to  the  chemist  it  is  the  substance 
that  is  of  importance.  To  a  zoologist  the  difference  between 
two  kinds  of  animals,  for  example  between  a  rabbit  and  an 
elephant,  is  most  marked ;  to  a  chemist,  on  the  other  hand, 
the  differences  are  of  small  importance,  since  both  creatures 
are  built  up  of  practically  the  same  chemical  substances,  and 
the  life  of  both  is  dependent  on  the  same  kind  of  chemical 
processes. 

Substances  are  continually  undergoing  change,  as  when 
coal  burns  or  wood  decays  or  the  food  of  animals  is  digested  ; 
and  all  such  changes  of  substances  come  within  the  scope  of 
chemistry.  Mere  changes  in  size  and  form,  as  when  a  block 
of  marble  is  chiseled  or  a  band  of  rubber  is  stretched,  have 
no  place  in  the  study  of  chemistry,  because  there  is  no  change 
of  substance. 

9 


10  :  '•  CBBMtCX&teffi  PHYSICAL  CHANGES 


1.  A  'Typical  Cher&ical  Substance:    Charcoal.     Let  us 
examine  a  familiar  substance,  charcoal.     Because  ordinary 
wood  charcoal  contains  impurities,  it  is  better  for  us  to  con- 
sider a  very  pure  form  of  charcoal,  which  may  easily  be  made 
by  charring  sugar. 

Charcoal  is  black,  porous,  light  in  weight,  brittle,  and  easily 
crushed  to  a  powder.  These  are  some  of  the  physical  prop- 
erties of  charcoal  and  they  are  inherent  in  charcoal.  It  is 
by  observing  these  properties  that  we  are  enabled  to  dis- 
tinguish it  from  other  substances  ;  for  example,  from  marble. 

When  charcoal  burns,  it  undergoes  a  great  change  and 
thereby  loses  all  of  these  physical  properties.  The  fact  that 
it  can  burn  when  in  contact  with  air  constitutes  one  of  the 
chemical  properties  of  charcoal. 

2.  A  Typical  Chemical  Change.    Let  us  place  a  layer  of 
sugar  charcoal  upon  a  grate  so  that  air  may  have  free  access 
to  it,  and  heat  the  lower  part  of  the  layer  until  it  begins  to 
glow  ;   in  other  words,  let  us  set  fire  to  it.     As  we  continue 
to  observe  the  charcoal  after  removing  the  source  of  heat, 
we  note  that  it  does  not  become  cool  again,  but  that  the  heat 
persists,   and  even  grows  stronger,   and  the  glow  spreads 
throughout  the  whole  layer.     We  note  that  the  bulk  of  the 
charcoal  is  diminishing,  and  at  length  we  find  that  it  has 
entirely    disappeared.     When    ordinary    wood    charcoal    is 
burned,  a  small  quantity  of  white  ashes  is  left,  otherwise  its 
behavior  is  exactly  the  same. 

What  now  is  the  real  nature  of  this  change?  Has  the 
charcoal  been  destroyed  ?  We  can  certainly  no  longer  see 
it  or  feel  it,  but  this  does  not  necessarily  mean  that  it  has 
ceased  to  exist  as  matter.  Many  well-known  forms  of  matter 
are  imperceptible  so  far  as  our  most-used  means  of  observa- 
tion go.  For  example,  when  we  look  through  a  pane  of  very 


A  TYPICAL  CHEMICAL  CHANGE  11 

clear  glass  or  a  layer  of  very  clear  water,  we  can  hardly  detect 
these  materials  in  our  line  of  vision.  We  live  at  the  bottom 
of  a  sea  of  air,  of  the  presence  of  which  we  have  abundant 
evidence  in  the  wind  and  in  such  facts  as  our  ability  to  use 
it  to  inflate  automobile  tires.  Moreover,  it  has  been  found 
by  scientific  men  to  be  an  invariable  rule  of  nature  that 
matter  is  never  destroyed.  This  rule  is  known  as  the  law 
of  the  conservation  of  matter.  Matter  may  change  its  form,  but 
its  quantity,  which  is  measured  by  its  weight,  never  increases  or 
decreases. 

If  we  accept  the  experience  of  scientific  men  as  proof  of 
the  correctness  of  this  law,  we  must  conclude  that  the  char- 
coal has  not  ceased  to  exist,  but  has  merely  changed  to  or 
become  part  of  a  substance  which,  like  air,  is  invisible.  In 
point  of  fact  this  conclusion  is  correct.  The  substance  pro- 
duced by  burning  the  charcoal  has  many  properties  by 
which  its  presence  may  be  ascertained.  One  of  its  most 
characteristic  properties  is  that  it  is  absorbed  by  limewater, 
and  thereby  at  once  renders  the  latter  milky  in  appearance. 
This  is  a  property  not  possessed  by  air  or  by  charcoal.  If 
this  product  of  burning  charcoal  is  collected  and  weighed, 
it  is  found  to  weigh  three  and  a  half  times  as  much  as  the 
original  charcoal. 

Assuming  again  the  correctness  of  the  law  of  the  conser- 
vation of  matter,  which  states  also  that  the  quantity  of 
matter  does  not  increase,  we  must  conclude  that  this  gas 
that  is  formed  cannot  be  composed  alone  of  the  same  matter 
as  the  charcoal,  but  that  something  must  have  been  added 
to  that  substance.  This  added  substance  must  have  come 
from  the  air.  It  is  in  fact  what  we  call  oxygen,  and  the 
new  substance  formed  by  its  action  with  the  charcoal  is 
called  carbon  dioxide. 


12  CHEMICAL  AND   PHYSICAL  CHANGES 

That  something  from  the  air  is  essential  in  the  production 
of  the  new  substance  is  all  the  more  strikingly  shown  if  the 
charcoal  is  heated  out  of  contact  with  the  air ;  as,  for  ex- 
ample, in  a  vacuum,  that  is,  in  a  closed  vessel  out  of  which 
the  air  has  been  pumped.  When  the  charcoal  is  again  allowed 
to  cool  after  this  heating,  it  recovers  its  original  appearance 
and  is  found  to  have  neither  lost  nor  gained  in  weight. 

Let  us  sum  up  the  facts  already  stated  concerning  the 
behavior  of  charcoal  and  air.  When  charcoal  burns  in  air, 
the  charcoal  ceases  to  exist  as  charcoal,  but  it  and  some- 
thing from  the  air  form  a  new  substance,  which  has  proper- 
ties different  from  those  of  either  of  the  original  substances. 
Accompanying  this  change  in  the  kind  of  substance,  a  large 
amount  of  heat  is  developed. 

3.  Characteristics  of  a  Chemical  Change.     In  the  example 
which  we  have  been  considering  we  have  illustrated  some  of 
the  important  characteristics  of  chemical  changes,  and  we 
should  now  be  able  to  recognize  a  case  of  chemical  change  by 
observing  these  characteristics.      The  substances  present  in 
the  beginning  lose  their  properties  and  are  changed  into  new 
substances  with  radically  different  properties.     In  general,  heat 
effects  accompany  chemical  changes  and  these  effects  are  usually 
wry  marked  when  the  change  takes  place  rapidly.     The  heat 
may  be  less  intense,  however,  than  in  the  case  of  the  burning 
charcoal.     Some  chemical  changes  absorb  heat  rather  than 
give  it  off. 

4.  Examples   of  Chemical  Change.     All   other  cases   of 
burning  are  examples  of  chemical  change;    for  in  all  such 
cases  —  whatever  the  fuel  that  is  burnt  —  new  substances 
with  new  properties  are  formed    and    the    old    substances 
disappear.      Moreover  the  change  is  accompanied   by  the 
active  giving  forth  of  heat.    The  explosion  of  gunpowder 


PHYSICAL  CHANGE  13 

and  of  dynamite  (see  frontispiece)  are  also  instances  of 
chemical  change. 

The  rusting  of  iron,  the  souring  of  milk,  the  decaying 
of  wood,  and  the  digestion  of  food  in  the  body  may  be  men- 
tioned as  other  familiar  cases  of  chemical  change.  In  these 
latter  cases  the  rate  of  change  is  slower  than  in  burning  and 
the  heat  effects  are  not  so  apparent.  Nevertheless,  it  is 
true  that  the  animal  body  is  kept  warm  by  the  chemical 
changes  which  the  food  undergoes. 

5.  Physical  Change.  We  observe  changes  of  some  of  the 
properties  when  a  block  of  marble  is  broken  into  fragments, 
or  a  steel  spring  is  compressed,  or  the  filament  of  an  incan- 
descent electric  lamp  is  heated  to  white  heat  by  the  current, 
or  gold  is  melted,  or  water  is  vaporized  over  a  fire  or  is  frozen 
on  a  cold  day.  But  the  steel  spring  returns  to  its  original 
shape  when  it  is  released;  on  turning  off  the  current,  the 
incandescent  filament  again  grows  cold  and  black ;  on  cool- 
ing, the  melted  gold  resolidifies  and  the  water  vapor  con- 
denses to  liquid  water;  and,  on  returning  to  the  original 
temperature,  the  ice  again  turns  to  water.  The  fragments 
of  the  shattered  block  of  marble,  it  is  true,  do  not  reunite 
into  one  solid  piece,  but  this  is  not  essential,  because  each 
of  the  fragments  is  recognized  at  once  as  marble,  since  it 
possesses  the  properties  of  marble  and-  differs  only  in  size 
and  shape  from  the  original  block. 

Such  changes  involve  no  change  in  the  chemical  nature  of 
the  substances  and  they  are  known  as  physical  changes. 

In  our  study  of  chemistry  we  shall  have  frequent  occasion 
to  distinguish  between  chemical  and  physical  changes,  and 
in  most  cases  it  is  a  very  simple  matter  to  make  the  distinc- 
tion. There  are,  however,  cases  on  the  border  line  concern- 
ing which  even  scientists  fail  to  agree. 


14  CHEMICAL  AND   PHYSICAL  CHANGES 

SUMMARY 

Chemistry  is  the  science  which  deals  with  substances  and  the  changes 
which  substances  undergo  and  are  capable  of  undergoing. 

A  chemical  change  takes  place  when  a  substance,  or  substances, 
undergoes  a  change  into  a  new  substance,  or  substances,  with 
widely  different  properties.  Chemical  changes  usually  involve 
the  production  of  or  the  absorption  of  a  large  amount  of  heat. 
The  burning  of  charcoal  is  one  of  the  best  known  and  most 
typical  of  chemical  changes. 

Conservation  of  Matter:  The  amount  of  matter  in  the  universe  is 
constant;  matter  is  not  created  or  destroyed.  This  statement 
is  known  as  the  law  of  the  conservation  of  matter. 

Physical  changes  are  those  in  which  bodies  of  substance  are  changed 
in  any  way  without  being  altered  in  their  chemical  nature. 

Questions 

1.  How  are  different  chemical  substances  distinguished  ? 

2.  State  three  ways  in  which  chemical  changes  differ  from  phys- 
ical changes. 

3.  Give  five  examples  of  physical  change. 

4.  Give  five  examples  of  chemical  change.     State  what  distin- 
guishes each  of  these  as  a  chemical  rather  than  a  physical  change. 

6.  Mention  three  instances  of  chemical  change  in  which  the  heat 
effect  is  so  intense  as  to  cause  emission  of  light. 

6.  Mention  three  instances  in  which  the  heat  effect  is  easily 
noticeable,  but  still  not  intense  enough  to  produce  a  glow. 

7.  Mention  three  instances  in  which  the  heat  effect  is  so  slight 
that  it  is  unnoticeable. 

8.  We  may  accept  it  as  a  reasonable  supposition  that  just  as 
much  heat  would  be  produced  by  the  complete  decay  of  a  piece  of 
wood  as  if  the  wood  were  to  be  burned.     How  do  you  explain  the 
fact  that  the  heat  is  very  intense  in  only  one  of  these  cases  ? 

9.  Name  the  products  of  "burning  coal  in  a  furnace.      Is  the 
chemical  reaction  carried  on  to  obtain  these  new  substances  ? 

10.   When  a  mason  slakes  lime  to  make  mortar,  is  he  interested 
in  the  heat  produced? 


CHAPTER  II 

MIXTURES   AND    CHEMICAL    SUBSTANCES 

6.  Mixtures.  Many,  in  fact  most,  of  the  bodies  of  matter 
familiar  to  us  in  everyday  life  are  mixtures.  Salt  water,  for 
example,  is  a  mixture  containing  water  and  salt.  A  glass  of 
salt  water  appears  as  clear  as  ordinary  pure  water,  and  it  is 
impossible  to  see  any  separation  into  individual  grains  of 
salt  or  drops  of  water.  Yet  we  know  that  we  can  make 
salt  water  by  dissolving  salt  in  water. 

Granite,  which  is  a  well-known  building  stone,  is  also  a 
mixture,  —  usually  of  three  substances,  quartz,  feldspar, 
and  mica;  but  in  granite  the  separate  particles  of  these 
substances  are  large  enough  to  be  easily  seen. 

Ordinary  garden  soil  is  a  very  complex  mixture,  containing 
sand,  particles  of  decaying  leaves,  water,  and  various  salts 
such  as  potassium  nitrate,  potassium  carbonate,  and  calcium 
phosphate.  The  ingredients  of  the  soil  are  so  thoroughly 
mixed  that  they  cannot  be  identified  with  the  naked  eye. 

Many  of  the  materials  which  are  brought  to  the  practical 
chemist  for  "  analysis  "  are  very  complicated  mixtures,  and 
the  work  of  the  chemist  frequently  consists  in  finding  out 
what  substances  are  present  in  the  mixtures  and  usually 
also  what  quantity  of  each  substance.  This  he  is  able  to 
do  because  he  has  been  trained  to  know  pure  substances  by 
their  properties  and  has  learned  to  use  systematic  methods 
of  separating  these  substances. 

B.    AND    W.    CHEM, 2         15 


10  MIXTURES  AND   CHEMICAL  SUBSTANCES 

7.  Chemical  Substances.     Any  kind  of  matter  which  is 
not  a  mixture  is  a  chemical  substance.     Salt,  sugar,  sulphur, 
and  carbon  (charcoal)  are  examples  of  chemical  substances, 
and  none  of  these  can  be  produced  like  salt  water  or  garden 
soil  by  the  mere  putting  together  of  different  ingredients. 

A  chemical  substance  is  uniform  throughout  its  mass. 
Thus  if  we  break  a  piece  of  any  one  of  the  above-mentioned 
substances  into  countless  small  pieces  and  examine  each  of 
these  pieces  by  any  means  at  our  disposal,  we  find  that  each 
of  the  minute  particles  has  just  the  same  properties,  except 
those  of  size  and  shape,  as  all  the  other  pieces  or  as  the  un- 
broken lump. 

But  mere  uniformity  throughout  the  mass  of  a  body  of 
matter  does  not  necessarily  show  that  it  is  a  single  substance, 
for  some  mixtures,  like  salt  water,  are  as  uniform  as  the 
substances,  salt,  sugar,  sulphur,  and  charcoal.  Yet  we  know 
that  we  can  prepare  salt  water  by  mixing ;  we  find  further- 
more that  we  are  able  to  separate  salt  water,  as  well  as 
other  mixtures,  by  rather  simple  processes,  a  few  of  which 
are  outlined  in  the  next  section. 

8.  Methods  of  separating  mixtures  into  their  components 
are  based  on  some  physical  property  by  which  the  compo- 
nents differ  markedly  from  each  other.     Thus  a  component 
may  be  attracted  to  a  magnet  and  the  other  components 
not;    a  component  may  be   very  volatile   or  very  soluble 
or  very  heavy.      Methods  of  separation  are  as  varied  as 
physical  properties  are  varied,  but  we  shall  describe  only  a 
few  practical  methods  of  separating  some  typical  mixtures. 

A  block  of  granite  might  be  crushed,  and  then  with  the 
aid  of  a  magnifying  glass  and  delicate  forceps  the  separate 
particles  of  quartz,  feldspar,  and  mica  might  be  picked  out 
and  laid  in  separate  piles.  This  method,  which  may  be 


MECHANICAL  SEPARATION 


17 


called  simple  mechanical  separation,  would,  however,  prove 
very  wearisome. 

Salt  water  is  a  mixture  of  so  intimate  a  character  that  the 
most  powerful  microscope  fails  to  show  any  separate  particles 
of  salt  and  of  water,  yet  this  mixture  may  be  separated  by 
distilling.  The  salt  water  is  placed  in  a  flask  and  boiled ; 
the  water  changes  into  a  vapor,  steam,  which  passes  off  and 
may  be  led  through 
a  condenser,  that  is, 
a  pipe  surrounded  by 
a  cooling  bath.  Here 
the  steam  condenses 
to  water  which  drips 
from  the  lower  end 
of  the  condenser  tube 
(see  Fig.  1).  The 
salt,  which  is  not 
volatile,  is  left  behind 
in  the  flask  as  a  mass 


FIG.  1.  —  Distilling  Apparatus.  A,  flask  in 
which  liquid  is  boiled.  B,  trap  to  catch  spray 
from  boiling  liquid  and  prevent  any  non- 
volatile matter  passing  over  with  the  vapor. 
C,  condenser.  D,  receiver.  E,  cooling  water 


of  crystals.  This 
method  of  distillation 
has  a  very  wide  prac-  e"ters  ufrom  *»?•  .f  •  °/erflow-  <*> 

through  which  liquid  returns  to  flask. 

tical  use  for  separat- 
ing mixtures  of  volatile  and  non-volatile  substances,  or  even 
for  separating  substances  of  different  degrees  of  volatility. 

A  mixture  of  sugar  and  sand  can  be  separated  if  it  is 
stirred  up  with  water;  the  sugar  dissolves  in  the  water, 
while  the  sand  remains  unchanged.  By  pouring  off  the  solu- 
tion of  sugar  from  the  sand  one  accomplishes  the  separation. 
The  solution  may  be  made  to  give  up  its  sugar  if  the  water 
is  evaporated  away,  just  as  the  salt  water  gave  up  its  salt 
on  distillation.  Here  separation  is  accomplished  by  taking 


18  MIXTURES  AND   CHEMICAL  SUBSTANCES 

advantage  of  differences  in  solubility.  Such  separations  are 
carried  out  on  an  enormous  scale  in  practical  work. 

A  mixture  of  light  and  heavy  particles,  as,  for  example, 
a  mixture  of  powdered  sulphur  and  iron  filings,  may  be 
separated  by  taking  advantage  of  differences  in  density. 
Neither  of  these  substances  dissolves  in  water;  but  if  the 
mixture  is  stirred  vigorously  with  a  large  amount  of  water, 
the  whole  becomes  suspended  in  the  liquid.  If  the  liquid 
is  then  left  a  moment  at  rest,  the  particles  of  iron,  being  heavy, 
settle  rapidly  to  the  bottom,  while  the  lighter  particles  of 
sulphur  remain  suspended  for  a  longer  time.  Then  if  the 
upper  part  of  the  liquid  is  poured  off  at  the  right  moment, 
most  of  the  suspended  sulphur  is  carried  with  it,  whereas 
the  iron  is  left  behind.  If  this  process  be  carried  out  with 
sufficient  care,  the  sulphur  can  be  completely  separated 
from  the  iron. 

This  type  of  separation  is  frequently  made  use  of  in  indus- 
trial and  mining  operations  that  are  carried  out  on  a  large 
scale.  Most  interesting  perhaps  is  the  separation  of  gold 
dust  from  gold-bearing  sand. 

9.  Chemical  Substances  that  can  be  separated.  As 
already  said  above,  a  chemical  substance  is  a  material  which 
cannot  be  prepared  by  the  mere  mixing  of  different  ingredi- 
ents, nor  can  it  be  separated  by  non-chemical  means  into 
different  kinds  of  matter.  Now  the  great  majority  of  chemi- 
cal substances,  for  example  salt  and  sugar,  can  be  separated 
chemically  into  two  or  more  different  substances,  but  the 
chemical  means  that  must  be  employed  to  do  this  are  very 
unlike  the  methods  described  under  the  last  heading.  To 
break  down  a  chemical  substance  into  different  substances 
generally  involves  the  expenditure  of  a  large  amount  of 
energy,  perhaps  in  the  form  of  intense  heat,  perhaps  in  the 


SUMMARY  AND  QUESTIONS  19 

form  of  an  electric  current,  and  perhaps  as  chemical  energy 
through  interaction  with  some  other  chemical  substance. 

Furthermore,  when  a  pure  substance  is  separated  chemi- 
cally, the  new  substances  are  totally  different  in  their  proper- 
ties from  the  original  substance;  whereas  when  a  mixture 
is  separated,  each  of  the  components  still  possesses  the  same 
properties  which  it  showed  in  the  mixture. 

SUMMARY 

A  mixture  is  composed  of  two  or  more  chemical  substances.  A 
coarse-grained  mixture  may  be  laboriously  separated  into  its 
components  by  sorting  out  the  particles  by  hand.  More  in- 
timate mixtures  may  be  separated  by  taking  advantage  of 
differences  in  the  physical  properties  of  the  components,  as, 
for  example,  differences  in  volatility,  solubility,  or  density. 

A  pure  chemical  substance  consists  of  a  single  kind  of  matter.  If 
a  pure  substance  is  capable  of  being  separated  chemically 
into  different  substances,  it  is  only  by  a  process  totally  dif- 
ferent from  the  methods  of  physical  separation  of  mixtures; 
moreover  the  separation  yields  totally  new  substances  with 
altogether  new  properties. 

Questions 

1.  Look  up  in  the  dictionary  the  words  matter,  substance,  mixture, 
body,  object,  and  decide  which  of  the  meanings  applies  in  chemistry. 

2.  Distinguish  between  the  popular  meaning  of  the  word  sub- 
stance and  the  scientific  meaning. of  the  term  pure  substance. 

3.  Make  use  of  the  terms  substance,  mixture,  body,  and  object  in 
classifying   the    following:    rain  water,    sea   water,   the   Atlantic 
Ocean,  Lake  Superior,  the  block  of  ice  that  the  ice  man  just  de- 
livered, sugar,  maple  sugar,  hash,  a  mince  pie,  the  marble  statue 
in  front  of  the  city  library,  the  ledge  of  marble  on  the  mountain, 
iron,  the  old  cannon  in  the  museum. 

4.  How  could  the  property  of  magnetism  be  used  to  separate  a 
mixture  of  iron  filings  and  powdered  sulphur? 

6.    How  could  salt  be  separated  from  sand? 


CHAPTER   III 
ELEMENTS    AND    COMPOUNDS 

10.  Elements.  Of  the  substances  of  which  we  have 
spoken  in  the  previous  chapter,  some  —  namely,  sulphur, 
charcoal,  and  iron  —  cannot  be  separated  into  other  than 
just  the  single  substances  by  any  means  known  to  us,  either 
by  physical  or  by  chemical  processes.  Such  substances  are 
known  as  elements ;  and  up  to  the  present  day  about  eighty 
of  these  elementary  substances  have  been  discovered  (see 
table  inside  the  back  cover). 

The  elements  may  combine  chemically  with  other  ele- 
ments and  thereby  form  totally  different  substances,  but 
the  elements  can  always  be  separated  again  undiminished 
in  weight  from  the  compounds.  For  example,  if  one  kilo- 
gram of  charcoal  burns,  although  the  charcoal  disappears, 
there  is  still  one  kilogram  of  carbon  in  the  carbon  dioxide 
which  is  formed.  (Carbon  is  the  name  of  the  element  of 
which  the  substance  charcoal  consists.  The  element  is 
always  carbon,  whatever  the  form  or  state  of  chemical  com- 
bination; charcoal  is  a  form  of  the  uncombined  element.) 
The  carbon  dioxide  which  escapes  into  the  air  is  taken  up 
mostly  by  trees  and  plants,  where  the  element  undergoes 
further  chemical  change,  and  thus  this  kilogram  of  carbon 
may  eventually  become  part  of  the  wood  of  trees.  The 
wood  may  be  charred  again,  and  thus  some  at  least  of  the 
original  charcoal  will  return  again  to  the  form  of  charcoal. 

20 


CHEMICAL   COMPOUNDS  21 

Whatever  the  final  form  of  the  one  kilogram  of  carbon,  it 
still  exists  somewhere  unchanged  in  weight. 

We  are  perhaps  not  warranted  in  claiming  with  certainty 
that  the  elements  are  absolutely  the  simplest  forms  of  matter. 
All  that  the  chemist  means  to  imply,  when  he  calls  a  sub- 
stance an  element,  is  that  it  is  something  which  human  skill 
has  been  unable  to  resolve  into  two  or  more  different  sub- 
stances.1 In  the  past  men  have  regarded  as  elements  many 
substances  which  have  since  been  shown  to  be  separable. 

One  of  the  early  attempts  to  classify  substances  was  that 
of  the  ancient  Greek  philosophers,  under  the  leadership  of 
Aristotle,  who  divided  all  substances  into  four  supposed 
elements,  —  earth,  air,  fire,  and  water.  This  division  was 
based  on  fanciful  speculation  instead  of  scientific  knowledge ; 
but  within  recent  times,  even  within  a  few  decades,  what 
have  been  supposed  to  be  elements  have  in  a  few  instances 
been  found  to  be  really  mixtures  of  two  or  more  elements. 

11.  Chemical  Compounds.  The  number  of  chemical  sub- 
stances which  the  chemist  has  discovered  is  almost  countless. 
A  few  of  them,  as  we  have  seen,  are  elements,  but  the  great 
majority  are  compounds  of  two  or  more  elements.  Salt 
and  sugar  are  chemical  compounds,  so  also  are  water,  baking 
soda,  washing  soda,  and  alum. 

It  must  not  be  thought,  however,  that  these  compounds 
are  any  the  less  chemical  substances  than  the  elements  car- 

1  Recent  discoveries  in  connection  with  the  study  of  radio- 
active elements  such  as  radium  have  tended  to  show  that  some 
of  the  elements,  perhaps  all,  are  constantly  undergoing  changes, 
some  being  produced  while  others  are  being  disintegrated.  It  may 
be  that  the  agencies  which  have  produced  the  elements  are  still  at 
work.  Even  with  the  most  rapidly  changing  element,  however, 
any  increase  or  decrease  in  quantity  is  so  small  that  it  is  almost  or 
quite  beyond  the  range  of  detection  with  the  chemical  balance. 


22  ELEMENTS  AND   COMPOUNDS 

bon,  sulphur,  and  iron,  for  compounds  have  just  as  charac- 
teristic properties  as  have  the  elements  themselves,  and  these 
properties  are  lost  altogether  whenever  the  compounds  are 
separated  into  their  elements. 

Another  important  characteristic  of  chemical  compounds 
is  that  they  always  contain  exactly  the  same  proportion  by 
weight  of  their  constituent  elements.  This  statement  is  known 
as  the  law  of  definite  proportions.  Mixtures,  on  the  other 
hand,  may  contain  variable  proportions  of  their  ingredients. 
We  shall  find  in  succeeding  chapters  that  a  knowledge  of  the 
definite  proportions  in  which  elements  are  united  in  com- 
pounds is  a  matter  of  the  utmost  importance  in  the  develop- 
ment of  chemical  science. 

12.  Mixtures  and  Compounds.     Let  us  try  to  make  clear 
the  difference  between  physical  mixtures  and  chemical  com- 
pounds by  comparing  the  effect  of  merely  mixing  two  chemi- 
cal substances  on  the  one  hand  and  causing  them  to  com- 
bine chemically  on  the  other  hand. 

Mixing  may  be  accomplished  by  merely  shaking  or  stirring 
substances  together  if  they  are  first  very  finely  divided. 

13.  Mixture    of   Iron  and    Sulphur.     A  mixture  which 
can  readily  be  made  in  the  laboratory  is  one  of  iron  and 
sulphur.     If  the  very  finest  powdered  sulphur  and  an  irn- 
palpably  fine    iron  dust    are  thoroughly  stirred    together, 
a  mixture  is  obtained  which  appears  like  a  uniform  gray 
powder  ;    to  discover  any  unevenness  in  its  texture  it  is 
necessary    to   employ    a    powerful    microscope.      Still    this 
mixture  may  be  separated  into  sulphur  and  iron   by  the 
method  described  on  page  18.     Another  convenient  method 
by  which  it  can   be  separated  is  to  treat  it  with  carbon 
disulphide,  a  liquid  in  which  sulphur  dissolves  much  as  salt 
dissolves  in  water.     The  iron  is  left  undissolved  and  may 


COMPOUND  OF  IRON   AND   SULPHUR  23 

be  separated  by  pouring  off  the  liquid,  and  the  sulphur 
may  be  obtained  from  the  solution  by  allowing  the  carbon 
disulphide  to  evaporate. 

14.  Compound  of  Iron  and  Sulphur.  Now  let  us  fill 
a  test  tube  with  some  of  this  intimate  mixture  of  sulphur 
and  iron,  and  heat  the  mixture  in  one  spot  until  it  begins 
to  glow.  On  removing  the  test  tube  from  the  flame  the 
glow  does  not  subside,  but  spreads  to  every  part  of  the 
mixture,  giving  forth  light  and  heat  and  thus  furnishing 
abundant  evidence  that  a  vigorous  change  is  taking  place. 
After  the  whole  mass  has  thus  entered  into  this  reaction 
and  has  afterward  been  allowed  to  cool,  it  is  found  that  there 
remains  a  hard  cake.  If  proper  proportions  of  iron  and  of 
sulphur  were  used  this  cake  may  now  be  pulverized  and  stirred 
up  with  water,  and  we  shall  not  find  any  lighter  particles  of 
sulphur  which  float  off  nor  any  heavier  particles  of  iron 
which  settle.  Neither  will  it  be  possible  to  dissolve  put 
sulphur  by  means  of  carbon .  disulphicje  and  leave  iron 
behind. 

Here,  again,  a  chemical  reaction  has  occurred  with  the 
appearance  of  a  good  deal  of  heat  and  light  energy.  A  new 
substance  totally  unlike  either  of  the  original  substances  has 
been  formed,  and  this  cannot  now  be  separated  into  the 
two  original  substances  by  any  of  the  comparatively  simple 
methods  by  which  mixtures  of  the  latter  substances  were 
separated. 

The  study  of  chemistry  is  vitally  concerned  with  chemical 
compounds,  but  it  is  concerned  to  a  much  smaller  extent 
with  mixtures.  For  in  mixtures  there  is  no  real  associa- 
tion of  the  component  substances  and  a  study  of  the  prop- 
erties of  the  mixture  is  only  a  study  of  the  properties  of 
the  separate  substances.  But  in  compounds,  on  the  other 


24  ELEMENTS  AND   COMPOUNDS 

hand,  the  characteristics  of  the  constituent  substances  are 
lost  and  the  compounds  themselves  are  distinctive  chemical 
individuals  with  distinctive  properties  of  their  own. 


SUMMARY 

An  element  is  a  substance  which  cannot  be  resolved  into  different 

chemical  substances  by  any  agency  under  human  control. 

All  the  matter  of  which  our  earth  is  composed  is  made  up  of 

about  eighty  elements. 
Chemical  compounds  are  pure  substances  which  can  be  separated 

by  chemical  means  into  two  or  more  elementary  substances. 

Chemical  compounds  are  characterized  by  a  definite  ratio  by 

weight  of  their  constituent  elements. 

Questions 

1.  Define  element ;  compound. 

2.  Name  five  common  elements. 

3.  Name  five  common  compounds. 

4.  In  the  light  of  our  present  knowledge  of  the  elements,  can  we 
ever  hope  to  be  able  to  convert  lead  into  gold? 

5.  What  is  true  of  the  proportions  of  the  constituents  of  a  com- 
pound that  is  not  true  of  the  components  of  a  mixture? 


CHAPTER  IV 
COMBUSTION 

THE  chemical  phenomenon  most  familiar  in  everyday 
life,  in  fact  so  familiar  that  the  average  person  hardly 
realizes  that  it  has  any  connection  with  chemistry,  is  ordinary 
burning  or  combustion.  Combustion  is,  however,  not  only 
the  most  common,  but  also  one  of  the  most  important,  of 
chemical  reactions,  since  it  is  the  energy  liberated  in  this 
reaction  which  furnishes  the  human  race  with  most  of  its 
heat,  light,  and  power. 

15.  Historic  Views  as  to  the  Nature  of  Combustion.  Com- 
mon as  is  the  phenomenon  of  fire,  it  is,  until  understood, 
most  mysterious.  Fire  was  regarded  by  the  ancients  as  one 
of  the  gifts  of  the  gods.  By  the  Greek  philosophers  it  was 
regarded  as  one  of  the  four  elements  of  which  the  world  is 
composed.  At  a  later  period  it  was  regarded  as  consisting 
in  the  escape  of  a  volatile  substance  called  phlogiston,  which 
was  supposed  to  be  contained  in  all  substances  capable  of 
burning.  It  was  only  during  the  last  of  the  eighteenth  cen- 
tury and  in  the  beginning  of  the  nineteenth  that  the  true 
chemical  nature  of  combustion  gradually  came  to  be  appre- 
ciated. This  was  scarcely  more  than  a  century  ago,  and 
since  combustion  is  one  of  the  most  important  of  chemical 
phenomena  and  since  an  explanation  of  its  nature  first  led 
to  an  understanding  of  other  chemical  phenomena,  it  is 
plain  that  chemistry  is  still  a  comparatively  young  science. 

25 


26  COMBUSTION 

16.  Modern  Theory  of   Combustion.     Combustion   con- 
sists of  a  chemical  uniting  of  the  combustible  substance  with 
oxygen,  a  substance  which  is  one  of  the  components  of  the 
air;    the  product  of  this  union  is  in  many  cases  a  gaseous 
substance,  but  in  all  cases,  whether  the  combustion  product 
be  a  solid,  a  liquid,  or  a  gas,  its  weight  is  exactly  equal  to 
the  weight  of  the  substance  burned  plus  the  weight  of  the 
oxygen  consumed. 

17.  Growth  of  Modern  Theory  of  Combustion.     Let  us 
outline  a  few  of  the  important  facts  regarding  combustion 
and  recount  some  of  the  important  steps  by  which  the  modern 
view  of  the  nature  of  combustion  was  developed. 

Wood,  coal,  and  charcoal,  if  kindled,  that  is  if  heated  suffi- 
ciently so  that  active  combustion  begins,  will  burn,  and 
what  we  call  fire  is  observed.  Flames  which  are  hot  and 
luminous  arise  and  then  seem  to  vanish  into  the  air. 

Combustion  is  not  limited,  however,  to  those  cases  where 
substances  burn  vigorously  with  a  flame.  Even  the  earlier 
chemists  who  upheld  the  phlogiston  theory  realized  that  the 
burning  of  wood  and  the  rusting  of  iron  were  phenomena  of  a 
similar  nature.  Their  mistake  lay  in  thinking  that  in  each 
case  there  was  an  escape  of  phlogiston.  When  iron  rusted, 
they  thought  that  phlogiston  escaped,  and  that  the  rust 
left,  or  the  calx,  as  it  was  then  termed,  was  iron  which  had 
lost  the  phlogiston  which  belonged  to  it.  We  now  know 
that  iron  rust  is  produced  by  the  slow  combining  of  iron 
with  oxygen,  and  that  the  weight  of  the  rust,  or  oxide,  is 
exactly  equal  to  the  sum  of  the  weights  of  the  iron  and  the 
oxygen  consumed. 

Much  iron  oxide  is  found  in  the  earth  as  a  brown  or  black 
ore,  and  to  obtain  the  metal  this  ore  is  heated  in  a  furnace 
with  charcoal  or  coke  which  removes  the  oxygen  from  the 


OXIDATION  27 

oxide  and  leaves  uncombined  iron.  This  process  for  obtain- 
ing iron  from  its  ore  is  a  very  ancient  one,  and  was  well 
known  to  the  phlogistic  chemists,  but  they  imagined  that 
charcoal  was  pure  phlogiston  and  that  it  combined  with  the 
calx  (iron  oxide)  to  produce  the  metal,  which  they  regarded 
as  the  compound  of  the  calx  and  phlogiston.  They  failed  to 
appreciate  and  correctly  interpret  the  fact  that  the  calx 
weighed  more  than  the  metal. 

18.  Use  of  the  Balance.     When,  however,  the  balance 
came  into  general  use  as  a  necessary  instrument  in  the  study 
of  chemistry  and  the  fact  that  metal  oxides  weigh  more  than 
the  metals  from  which  they  are  formed  became  better  under- 
stood, the  old  phlogistic  idea  of  combustion  gave  way  in 
favor  of  the  modern  theory  of  oxidation. 

19.  Oxidation  of  Metals.     Iron  is  not  the  only  metal 
which  on  exposure  to  the  atmosphere  becomes  coated  with 
a  dull  non-metallic  layer.     In  fact,  most  of  the  better-known 
metals,  with  the  exception  of  the  precious  metals,  gold,  silver, 
and  platinum,  are  soon  tarnished  or  corroded  to  a  greater 
or  less  degree,  depending  on  their  chemical  activity  and  on 
whether  the  coating  first  formed  is  permeable  or  impermeable 
to  the  gases  of  the  air. 

Lead,  copper,  and  zinc,  for  example,  when  freshly  polished, 
display  bright  metallic  surfaces;  but  after  exposure  to  the 
air,  as  when  they  are  used  for  gutters  or  for  trimmings  on 
roofs,  lead  becomes  covered  with  a  gray  and  lusterless  coat- 
ing, copper  with  a  green  coating,  and  zinc  with  a  dull 
whitish  coating.  If  instead  of  being  exposed  to  the  weather 
these  metals  are  heated  in  contact  with  the  air,  a  more  rapid 
change  occurs.  Lead,  if  it  is  kept  above  its  melting  tem- 
perature and  stirred  so  as  to  have  a  fresh  surface  con- 
tinually exposed,  changes  in  a  fairly  short  time  to  a  dull  yellow 


28 


COMBUSTION 


powder.  Copper  when  heated  to  redness  becomes  covered 
with  a  black  or  reddish  crust  which  can  be  scraped  off  as  a 
brittle  scale.  Zinc  if  heated  to  a  high  temperature  catches 
fire  and  burns  with  an  intense  bluish  white  light  and  a  fine 
white  powdery  substance  (zinc  white)  is  formed.  In  all 
these  cases  it  was  found,  when  the  balance  was  used,  that 
the  product  of  the  combustion  was  heavier  than  the  original 

metal ;  furthermore 
it  was  observed  that 
when  a  metal  had 
once  been  heated  or 
charcoal  burned  in  a 
quantity  of  air  con- 
fined in  a  closed 
vessel,  the  residual 
air  had  no  further 
power  to  cause  cor- 
rosion or  burning. 
It  thus  became  ap- 
parent that  in  com- 
bustion some  com- 
ponent must  betaken 
from  the  air  to  com- 
bine with  the  com- 
bustible substance. 

20.  The  Products 
of  Combustion  of  a 
Candle.  It  can  be 
shown  by  the  balance 
that  there  is  an  in- 
crease in  weight,  not 
only  when  metals 


FIG.  2. — For  showing  that  the  Products  of  Com- 
bustion of  a  Candle  weigh  more  than  the  Candle  Itself. 
The  candle  is  placed  on  one  pan  of  the  balance 
and  above  it  is  suspended  a  glass  cylinder  filled 
with  sticks  of  sodium  hydroxide.  Weights  are 
then  placed  on  the  other  pan  to  exactly  counter- 
balance the  candle  and  absorption  cylinder. 
The  candle  is  now  lighted,  the  gases  from  the 
flame  pass  through  the  funnel  into  the  absorp- 
tion tube ;  the  pan  with  the  candle  soon  sinks, 
showing  that  it  is  growing  heavier.  To  keep  it 
in  balance  more  weights  must  continually  be 
added  to  the  other  pan  as  long  as  the  candle 
burns. 


DISCOVERY  OF  OXYGEN  29 

corrode  to  form  solid  oxides,  but  also  in  those  cases  in 
which  substances  burn  to  form  gaseous  products.  Thus, 
for  example,  a  balance  may  be  arranged  with  a  candle 
on  one  pan  and  suspended  above  it  a  lamp  chimney  con- 
taining sticks  of  sodium  hydroxide  to  absorb  the  products 
of  combustion  (Fig.  2),  and  the  whole  may  be  exactly 
counterpoised  by  placing  weights  on  the  other  pan. 
So  long  as  the  candle  remains  unlighted,  the  pans  remain 
balanced  and  neither  rises  or  falls.  When  the  candle  is 
lighted  and  begins  to  waste  away,  one  would  expect  the  pan 
holding  it  to  rise  because  of  a  decrease  in  weight.  What  is 
actually  seen  is  that  that  pan  sinks.  The  gaseous  combus- 
tion products  of  the  candle  have  been  caught  by  the  sodium 
hydroxide  and  these  are  thus  shown  to  weigh  more  than  the 
candle  alone. 

21.  Discovery  of  Oxygen  by  Priestley,  1774.  The  dis- 
covery of  pure  oxygen  by  Priestley  paved  the  way  for  recog- 
nizing the  true  nature  of  combustion,  although  Priestley 
himself  was  unable  to  fit  together  the  facts  and  deduce  the 
true  solution. 

Mercury,  on  account  of  its  remarkable  properties,  was 
always  a  favorite  substance  for  experimentation  with  the 
early  chemists.  It  had  been  known  that  if  this  metal  was 
maintained  at  a  temperature  just  below  its  boiling  point 
(considerably  below  a  red  heat)  it  slowly  changed  into  a 
bright  red  powder  —  the  calx  of  the  phlogistic  chemists. 
Priestley  put  some  of  the  red  powder  in  the  top  of  a  glass 
tube  filled  with  mercury  and  placed  it  with  its  closed  end 
up  and  its  lower  end  sealed  by  being  dipped  in  a  vessel  of 
mercury.  By  focusing  the  sun's  rays  by  means  of  a  burn- 
ing glass  upon  the  red  powder  at  the  top  of  the  tube,  he  found 
that  a  gas  was  formed  which  forced  down  the  mercury  and 


30  COMBUSTION 

filled  some  little  space.  On  placing  a  lighted  candle  in  this 
gas,  he  found  that  it  burned  with  a  remarkably  brilliant 
flame  —  far  more  brilliant  than  when  it  burned  in  ordinary 
air.  He  was  much  mystified  by  this  behavior,  and  he  de- 
scribed the  gas  as  having  all  the  properties  of  ordinary  air 
only  in  much  greater  perfection.  He  called  it  dephlogisti- 
cated  air  and  regarded  it  only  as  very  pure  ordinary  air. 

22.  Lavoisier's  Explanation  of  Combustion.     It  remained 
for  the  French  chemist  Lavoisier  to  clearly  recognize  and 
state  the  true  nature  of  combustion.    He  connected  Priestley's 
discovery  with    the  fact    that  an    increase  rather   than  a 
decrease  in  weight  takes  place  when  substances  burn,  and 
with  the  fact  that  during  the  burning  the  air  not  only  loses 
in  volume,  but  also  loses  its  power  to  support  combustion. 

Lavoisier's  crucial  experiment  was  to  heat  mercury  within 
an  inclosed  vessel  of  air.  He  found  that  the  volume  of  the 
air  contracted  by  about  one  fifth,  while  at  the  same  time 
a  red  powder  formed  on  the  surface  of  the  mercury;  the 
residual  air  had  no  power  to  support  combustion.  After- 
wards, on  heating  this  red  compound  of  mercury,  a  gas  was 
given  off  whose  volume  was  one  fifth  that  of  the  original  air. 
This  gas  alone  supported  combustion  with  great  vigor,  and 
when  mixed  with  the  residual  four  fifths  of  the  original  air, 
restored  to  it  the  property  of  supporting  combustion  in  the 
ordinary  manner. 

23.  Combustion  in  the  Light  of  our  Present  Knowledge. 
The  atmosphere  consists  chiefly  of  a  mixture  of  two  gases,  — 
about  one  fifth  part  by  volume  is  oxygen,  the  supporter  of 
combustion;     about   four  fifths   is   nitrogen,  an   inert   gas 
which  plays  no  part  in  combustion  except  that  it  dilutes  the 
oxygen  and  decreases  the  intensity  of  its  action. 

When  substances  burn  in  the  air,  they  combine  chemically 


OXIDES  OF  CARBON  31 

with  the  oxygen  to  form  oxides,  while  the  nitrogen  is  left 
unchanged.  An  oxide  is  a  chemical  compound  of  an  element 
with  oxygen  and  nearly  all  of  the  elements  are  capable  of 
forming  oxides. 

24.  Oxides  of  Carbon.     Charcoal,  which  is  nearly  pure 
carbon,  burns  vigorously  when  it  has  a  plentiful  supply  of 
air  and  forms  carbon  dioxide,  an  invisible  gas.     If  all  or 
nearly  all  of  the  oxygen  of  the  air  is  used  up  in  this  way, 
the  latter  can  no  longer  support  combustion ;   neither  is  air 
that  is  so  deprived  of  its  oxygen  able  to  support  human  or 
animal  life.     Carbon  dioxide  is  not  actively  poisonous,  but 
the  absence  of  oxygen  results  in  suffocation. 

If  the  charcoal  is  burned  with  a  restricted  supply  of  air, 
then  a  part  or  even  the  whole  of  the  product  of  combustion 
consists  of  carbon  monoxide,  another  oxide  of  carbon  which 
contains  a  lesser  proportion  of  oxygen.  This  substance  is 
also  a  gas,  but  it  is  extremely  poisonous  when  breathed  by 
human  beings  or  animals.  It  is  due  to  the  formation  of 
this  gas  that  charcoal  fires  in  unventilated  rooms  have  been 
responsible  for  many  deaths. 

25.  Combustion  of  Wood.     Wood  is  a  compound  contain- 
ing carbon,   hydrogen,   and  oxygen.      When  it  burns  the 
chemical  reaction  is   not  unlike  that  of  burning  charcoal. 
The  existing  form  of  chemical  combination  of  the  wood  is 
broken  down  by  the  heat  of  the  fire,  and  the  carbon  and 
hydrogen  combine  with  oxygen  from  the  air  in  addition  to 
that  which  comes  from  the  wood  itself  and  form  carbon 
dioxide,  and  oxide  of  hydrogen  (water  vapor),  respectively. 

When  wood  decays,  it  undergoes  a  chemical  change  not 
essentially  different  from  that  of  burning  except  in  rapidity. 
The  heat  of  the  reaction  is  not  produced  fast  enough  to 
cause  flame  or  even  to  make  the  wood  feel  warm  to  the 

B.  AND  W.  CHEM. 3 


32  COMBUSTION 

touch,  but  still  the  products  of  the  reaction  contain  carbon 
dioxide  and  water. 

26.  Slow  Combustion  of  Foods  in  our  Bodies.  The  source 
of  the  body  warmth  and  muscular  energy  of  men  and  ani- 
mals lies  in  a  chemical  reaction  similar  to  that  of  the  burn- 
ing or  the  decay  of  wood.  The  burning  of  wood  is  a  violent 
reaction  and  produces  intense  local  heat ;  the  combustion  of 
foodstuffs  in  animals,  on  the  other  hand,  takes  place  slowly, 
like  the  decay  of  wood,  and  produces  only  the  degree  of 
warmth  necessary  to  keep  the  animal  in  health.  A  furnace 
fire  is  fed  with  coal  or  wood.  The  fire  in  a  man's  body  is  fed 
with  the  foods  that  contain  sugar,  starch,  or  fat.  Sugar, 
starch,  and  fat  have  a  chemical  composition  somewhat 
similar  to  that  of  wood ;  that  is,  they  are  composed  of  car- 
bon, hydrogen,  and  oxygen.  In  the  digestive  organs  they 
are  made  soluble  and  pass  into  the  blood  and  are  taken  by 
the  blood  into  all  the  tissues  of  the  body.  In  the  tissues,  to 
which  the  blood  also  brings  oxygen,  taken  from  the  air  that 
is  breathed  into  the  lungs,  there  takes  place  the  most  mar- 
velously  regulated  combustion.  This  combustion,  like  the 
active  burning  of  wood  in  the  air,  results  in  the  formation 
of  carbon  dioxide  and  water,  and  both  of  these  products  are 
exhaled  with  every  breath  from  the  lungs.  To  demonstrate 
that  water  vapor  is  present  in  the  air  from  the  lungs,  one 
has  only  to  breathe  against  a  cold  window  pane,  when  the 
glass  becomes  clouded  with  condensed  droplets  of  water ; 
and  to  show  the  presence  of  carbon  dioxide,  one  need  only 
blow  the  breath  through  a  glass  tube  into  a  tumbler  of  lime- 
water,  which  is  immediately  rendered  white  and  cloudy. 

When  we  exercise  violently,  our  breath  comes  much 
faster.  This  is  because  the  waste  or  combustion  of  bodily 
tissue  is  much  more  rapid  during  exercise  and  more  oxygen 


SMOKE   PREVENTION  33 

has  to  be  supplied  from  the  lungs  to  consume  it.  Also  it 
is  necessary  for  the  lungs  to  work  more  rapidly  in  order  to 
remove  the  greater  amount  of  carbon  dioxide  which  is 
produced. 

27.  Forced  Draft.     A  similar  case  of  the  need  of  an  in- 
creased supply  of  oxygen  when  more  rapid  combustion  is 
desired,  is  seen  in  the  use  of  the  blacksmith's  bellows.     In 
order  to  hasten  his  fire,  the  blacksmith  pumps  air  rapidly 
through  the  fire  by  means  of  the  bellows,  thus  supplying 
more  oxygen  and  hence  burning  more  fuel  in  a  given  time 
and  producing  a  hotter  fire. 

A  battleship  uses  a  forced  draft,  that  is,  it  pumps  air 
rapidly  through  the  fires,  when  attempting  to  attain  a  high 
speed.  The  principle  is,  of  course,  the  same  as  that  in- 
volved in  the  blacksmith's  fire ;  a  limited  amount  of  air  can 
burn  only  a  limited  amount  of  coal  and  thereby  produce 
only  a  limited  amount  of  heat.  By  supplying  more  air, 
more  coal  can  be  burned  and  more  heat  obtained. 

28.  Smoke   and  Smoke  Prevention.     It  is  not  an  easy 
matter  to  regulate  the  proper  supply  of  fuel  and  air  in  the 
fires  of  manufacturing    establishments,  or,  for  that  matter, 
in  other  fires,  and  as  a  consequence  our  manufacturing  cities 
are  to-day  burdened  with  the    smoke  nuisance.     The  ob- 
noxious black  smoke  which  issues  from  the  chimneys  consists 
in  the  main  of  unburned  fuel  material,  for  the  most  part  in 
the  solid  condition,  but  so  finely  divided  that  it  floats  for  a 
long  time  in  the  air  just  as  fine  dust  will  do. 

There  is,  however,  no  good  excuse,  now,  for  allowing  the 
smoke  nuisance  to  continue.  Modern  smoke-preventing 
devices  are  designed  in  the  first  place  to  regulate  the  supply 
of  both  the  air  and  the  fuel  so  that  neither  an  excess  of  fuel 
shall  be  wasted  as  smoke  nor  too  great  an  excess  of  air  shall 


34 


COMBUSTION 


enter  to  cool  off  the  fire  and  diminish  its  efficiency.  This 
regulation  is  usually  accomplished  by  means  of  some  form 
of  automatic  stoker  which  can  be  made  to  supply  the  fuel 
as  rapidly  or  as  slowly  as  desired,  while  the  air  supply  is 
controlled  by  a  blower.  In  the  second  place,  it  is  necessary 
that  the  smoke  which  does  form  shall  be  burned  before  it 
can  pass  into  the  cooler  flues  and  become  chilled  to  a  tem- 
perature below  its  burning  point.  This  is  accomplished  by 
admitting  some  fresh  air  into  the  chamber  above  the  fire 
and  allowing  this,  mixed  with  the  hot  gases  and  smoke,  to 
pass  through  a  considerable  distance  over  the  white-hot 
fire  bed  between  it  and  the  arch  of  very  hot  fire  brick  which 
forms  the  top  of  the  furnace.  In  this  way  the  smoke, 
together  with  sufficient  air,  is  kept  at  a  temperature  above 


FIG.  3.  —  Smoke  Nuisance  and  Smoke  Prevention.     Automatic  stoker  and  large- 
combustion  chamber  used  in  furnace  whose  chimney  shows  no  smoke. 

its  kindling  point  until  it  is  completely  burned.  Such  smoke 
consumers  are  now  largely  used,  and  allow  the  burning  of 
even  the  cheapest  grades  of  soft  coal  in  the  powdered  form 


SMOKE  PREVENTION 


35 


without  an  objectionable  amount  of  smoke  issuing  from  the 
chimney.  The  saving  due  to  the  economy  of  fuel,  in  many 
cases,  more  than  offsets  the  cost  of  the  smoke  prevention 
devices. 

S  i  r * 


WATfK  LfVCL  -^  A 

:  --t  ,—  _^  —^f^^t_=.    _~~**  _-  7  jFZ-fr^r"E^ 


FIG.  4. — Boiler  and  Furnace  with  Chain  Grate  Stoker. 


A.  Coal  chute. 

B.  Chain  grate. 

C.  Air  inlet.  M. 

D.  Damper.  N. 

E.  Flames.  O-P. 

F.  Course  of  heated  gases. 

G-H.  Baffle  walls.  Q. 

J.    Bridge  wall. 
K-L.   Zig-zag   passage    in    bridge 


wall  for  delivering  heated 
air  to  fire. 
Water  in  drum. 
Water  inlet. 
Outlet  and  inlet  for  water 

circulation. 
Water  tubes. 
R.   Blow-off  pipe. 
S.   Steam  outlet. 


36  COMBUSTION 

SUMMARY 

The  discovery  of  oxygen  by  Priestley  in  1774  was  the  first  step 
towards  understanding  the  nature  of  combustion.  Some- 
what later  Lavoisier  developed  the  present  explanation  by 
connecting  Priestley's  discovery  with  the  fact,  shown  with  the 
chemical  balance,  that  an  increase  in  weight  always  occurs 
during  combustion. 

Combustion  is  the  most  frequently  observed  as  well  as  the  most 
important  of  chemical  changes.  Combustion  consists  in  the 
uniting  of  oxygen,  the  active  component  of  the  air,  with  the 
combustible  substance.  In  rapid  combustion  great  heat  is 
developed  and  incandescence,  or  flame,  is  observed. 

Slow  combustion,  as  the  rusting  of  iron,  or  the  decay  of  wood,  is 
chemically  the  same  sort  of  a  process  as  rapid  combustion, 
but  the  heat  produced  has  time  to  escape  so  that  no  incan- 
descence, or  flame,  is  to  be  seen.  Food  is  the  fuel  for  the 
bodies  of  men  and  animals,  and  by  a  process  of  slow  combus- 
tion it  furnishes  animal  warmth  and  muscular  energy. 

Oxides.  Most  metals  give  solid  oxides  as  combustion  products. 
Carbon  gives  two  different  oxides :  with  a  plentiful  supply 
of  air,  carbon  dioxide,  a  comparatively  harmless  gas ;  with  a 
restricted  supply  of  air,  carbon  monoxide,  a  most  poisonous 
gas. 

Fuels  contain  carbon  as  their  most  important  constituent,  and 
besides  carbon  they  usually  contain  hydrogen.  Their  ma- 
terial combustion  products  are  principally  gases  and  are  of 
no  value.  They  are  burned  for  heat,  light,  and  power,  which 
are  essential  for  the  welfare  of  the  human  race. 

Smoke  consumption.  As  ordinarily  burned,  much  fuel  is  wasted 
as  smoke,  but  by  the  use  of  suitable  devices  this  waste  may 
be  prevented  and  the  nuisance  of  smoke  avoided. 

Questions 

1.  How  might  one  prove  that  something  from  the  air  is  con- 
cerned in  combustion? 

2.  Why  does  the  rusting  of  iron  continue  until  all  the  iron  is 
rusted,  whereas  the  tarnishing  of  copper,  zinc,  and  lead  is  on  the 
surface  only? 


QUESTIONS  37 

3.  Why,  then,  is  it  necessary  to  paint  iron  bridges,  although  it 
is  not  necessary  to  paint  copper-covered  roofs  ? 

4.  Explain  the  danger  from  charcoal  fires  in   closed  rooms. 
Why  is  there  not  the  same  danger  from  a  burning  gas  jet? 

5.  Give  some  evidence  to  prove  that  combustion  takes  place  in 
the  animal  body. 

6.  Discuss  from  a  common  viewpoint  the  rapid  breathing  of  a 
person  running  and  the  use  of  a  blacksmith's  bellows. 

7.  What  is  smoke?     Why  is  it  an  economy  to  prevent  it? 

8.  When  burning  dry  leaves,  if  one  rakes  a  lot  of  new  leaves 
all  at  once  on  top  of  the  fire  a  very  dense  smoke  is  formed.     Explain 
why  it  is  that  when  the  flame  finally  works  up  through  the  new 
leaves  and  bursts  out  through  the  top,  the  smoke  all  seems  to 
vanish. 

9.  Explain  why  in  unprogressive  manufacturing  establishments 
black  smoke  belches  from  the  chimney  when  the  stoker  opens  the 
furnace  door  and  shovels  a  quantity  of  soft  coal  on  the  fire.     How 
might  the  stoker  by  care  lessen  the  amount  of  smoke?     How  would 
you  design  and  run  a  furnace  to  prevent  smoke? 

10.  What  does  the  fact  that  a  pile  of  barnyard  manure  steams 
in  winter  indicate  is  taking  place  within  the  pile? 

11.  Why  does  a  camper  get  down  and  blow  his  fire  when  it  gets 
low? 

12.  Why  does  water  extinguish  a  fire? 


CHAPTER  V 

OXYGEN 

IN  the  last  chapter  it  has  been  seen  that  the  atmosphere 
contains  a  chemically  active  substance  which  is  called 
oxygen;  and  that  when  any  one  of  a  number  of  metals  is 
heated  in  the  air,  this  oxygen  combines  with  the  metal  to 
form  an  oxide.  It  was  by  heating  one  of  these  oxides,  namely 
oxide  of  mercury,  that  pure  oxygen  was  discovered  by 
Priestley. 

29.  Abundance.     Oxygen  is  now  known  to  be  the  most 
abundant  of  all  the  elements,  for  it  is  estimated  that  about 
one  half  of  the  whole  earth,  as  far  as  we  know  it,  is  composed 
of  oxygen.     Uncombined  oxygen  comprises  about  one  fifth 
of  the  air.     Eight  ninths  by  weight  of  water  is  oxygen,  which 
is  in  combination  with  hydrogen.     The  great  bulk  of  the 
solid  crust  of  the  earth  is  composed  of  compounds  of  oxygen ; 
thus  quartz  rock  and  ordinary  sand,  which  is  quartz  in  a 
fine  state  of  subdivision,  are  oxide  of  silicon,  containing  53 
per  cent  of  oxygen;    limestone  consists  of  two  oxides  in 
combination,  calcium  oxide  and  carbon  dioxide,  48  per  cent 
by  weight  of  the  limestone  being  oxygen. 

30.  Preparation   by   heating    Oxides.     The    method    of 
Pjiestley  for  the  preparation  of  oxygen  is  still  used  as  a 
convenient  laboratory  method  for  showing  that  oxygen  can 
be  generated  from  an  oxide;  it  is  never  used,  however,  to 
pi/epare  any  large  quantity  of  oxygen,  because  mercury  and 
its  compounds  are  too  costly. 

38 


PREPARATION  OF  OXYGEN 


39 


Many  less  expensive  oxides  will  evolve  oxygen  when 
heated  sufficiently,  but  in  most  cases  the  amount  of  heat 
required  is  so  great  that  the  process  is  too  costly  or  too 
inconvenient  on  that  account. 

31.  Preparation  from  Potassium  Chlorate.  For  the 
preparation  of  oxygen  on  the  small  scale  in  the  laboratory, 
recourse  is  had  to  potassium  chlorate,  which  contains  about 
40  per  cent  by  weight  of  oxygen  and  gives  it  all  off  when 
heated.  The  potassium  chlorate  is  mixed  with  its  own  bulk 
of  powdered  manganese  dioxide,  and  the  mixture  is  placed 
in  a  test  tube  (Fig.  5) . 
The  latter  is  provided 
with  a  delivery  tube 
leading  to  a  water 
trough,  and  on  gently 
heating  the  test  tube 
oxygen  gas  is  evolved 
and  can  be  collected 
in  test  tubes  or  in 
wide-mouthed  gas 
bottles  which  have 
been  previously  filled  with  water  and  inverted  in  the  trough. 

The  manganese  dioxide  and  the  potassium  chlorate  both 
contain  oxygen,  and  either  alone  will  give  up  oxygen  upon 
being  strongly  heated.  The  potassium  chlorate,  however, 
requires  to  be  heated  to  considerably  above  its  melting 
point  (334°  C.)  before  giving  up  its  oxygen  rapidly,  and  the 
manganese  dioxide  when  alone  yields  oxygen  only  when 
heated  to  a  red  heat.  If,  however,  the  two  substances  are 
mixed  together  and  heated,  oxygen  begins  to  come  off  at  a 
temperature  in  the  neighborhood  of  200°  C.  Hence  the 
mixture  is  invariably  used. 


FIG.  5. — Preparation  of  Oxygen.  Potassium 
chlorate  mixed  with  manganese  dioxide  is 
heated  and  the  gas  is  collected  over  water. 


40  OXYGEN 

32.  Catalysis.     If  the  separate  substances  are  carefully 
weighed  before  they  are  heated  and  the  products  left  after 
the  decomposition   are   separated   and  also  weighed,   it  is 
found  that  exactly  as  much  manganese  dioxide  remains  as 
was  taken  at  the  start,  while  in  place  of  the  potassium 
chlorate,  a  new  substance,  potassium  chloride,  is  left.     The 
latter  contains  no  oxygen,  and  it  is  clear  therefore  that  all 
of  the  oxygen  has  come  from  the  decomposition  of  the  potas- 
sium chlorate.     The  manganese  dioxide  has  suffered  no  ap- 
parent change,  but  has,  by  its  mere  presence,  helped  the 
other   substance   to   undergo    a   change    very   much   more 
smoothly  and  quickly  than  it  could  have  done  alone.     Cases 
like  this  we  shall  find  are  very  frequent  in  chemistry  and 
they  are  known  as  cases  of  catalysis.     The  catalytic  agent 
(in  this  case  the  manganese  dioxide)  does  not  cause  the  re- 
action, it  merely  makes  it  easier  for  it  to  take  place.     No 
more  does  lubricating  oil  cause  a  wheel  with  a  rusted  bearing 
to  turn,  it  simply  makes  it  easier  for  it  to  turn. 

33.  Preparation  from  Liquid  Air.     The  most  inexpensive 
source  of  oxygen  is  obviously  the  air,  and  comparatively 
recently  a  method  of  cheaply  separating  it  from  the  air  has 
been  perfected.     By  the  Linde  process,  the  air  is  first  liquefied 
by  high  compression   and  intense   cold,   and  the  liquefied 
air  within  the  apparatus  is  then  allowed  to  boil,  whereby 
nitrogen,  which  is  the  more  volatile  component  of  the  air, 
passes  off,  leaving  fairly  pure  oxygen.     The  latter  is  then 
allowed  to  vaporize  and  is  pumped  under  high  pressure  into 
steel  cylinders,  in  which  it  is  transported  and  sold. 

34.  Properties  of  Oxygen.     When  oxygen  prepared  by 
one  of  the  above  methods  is  examined,  it  is  found  to  be  a 
colorless,  odorless,  tasteless  gas  and  one  which  will  cause  a 
glowing  splinter  to  burst  into  a  vigorous  flame,  but  will  not 


PROPERTIES  OF  OXYGEN 


41 


itself  burn.  These  properties  can  be  discovered  without  the 
use  of  any  elaborate  apparatus  and  without  any  great 
degree  of  skill  in  observation.  If  we  were  to  obtain  a  gas 


FIG.  6.  —  Linde  Apparatus  for  making  Liquid  Air.  A,  air  inlet.  B,  com- 
pressing pump.  C,  cooling  coil  where  the  heat  developed  by  the  compres- 
sion of  the  air  is  withdrawn.  Coil  is  surrounded  by  running  cold  water. 
D,  inner  pipe  of  the  liquefying  coil ;  here  the  compressed  air  is  cooled  to  a 
very  low  temperature.  E,  outer  pipe  of  liquefying  coil ;  here  the  expanding 
air  cools  the  compressed  air  in  DD.  F,  expansion  chamber  and  reservoir 
for  liquid  air.  The  air  emerging  from  the  valve  at  lower  end  of  D  expands 
suddenly  and  produces  great  cold.  Part  of  the  air  is  thus  liquefied  and 
drops  into  the  reservoir  while  the  rest  passes  through  E  and  cools  the 
compressed  air  in  D  to  the  liquefaction  point. 

from  some  unknown  source  and  find  that  it  possessed  the 
properties  mentioned,  it  would  be  natural  to  assume  that  it 
was  oxygen.  There  is  at  least  one  other  gas,  however,  — 
and  there  might  be  more,  —  which  shows  approximately  the 


42  OXYGEN 

properties  mentioned,  and  it  might  be  possible  for  us  to 
obtain  this  gas  and  mistake  it  for  oxygen.  Hence  in  order 
to  distinguish  oxygen  with  certainty  from  all  other  sub- 
stances, we  must  define  its  properties  more  completely  and 
more  exactly. 

Properties  such  as  the  specific  gravity,  the  degree  of 
solubility  in  water,  and  the  temperature  of  liquefaction  and 
of  solidification  can  be  accurately  measured  and  stated  in 
exact  numerical  terms.  It  is  extremely  improbable  —  we 
can  even  say  impossible  —  that  a  substance  can  have  all 
the  easily  observed  properties  of  oxygen,  noted  earlier,  and 
give  exactly  the  same  quantitative  values  for  the  last-named 
properties,  and  still  be  any  other  substance  than  oxygen. 
Of  course,  if  the  substance  were  further  studied  and  foiind  to 
differ  from  oxygen  in  any  single  particular,  it  could  not  be 
oxygen ;  but  it  is  our  experience  with  nature  that  when  two 
substances  are  exactly  alike  in  several  of  their  properties 
they  will  be  found  to  be  alike  in  all  of  their  properties. 

35.  Density.     Oxygen   is   a   little  heavier   than   air,   its 
specific  gravity  referred  to  air  being  1.105,  that  is,  one  liter 
of  oxygen  weighs  1.105  times  as  much  as  one  liter  of  air  at 
the  same  temperature  and  under  the  same  pressure.     Ex- 
pressed in  other  terms,  one  liter  of  oxygen  at  0°   C.  and 
under  760  millimeters  pressure  (that  is  under  standard  con- 
ditions) weighs  1.43  grams;    that  is,  1000  cubic  centimeters 
of  oxygen  weigh  about  the  same  as  one  and  one  half  cubic 
centimeters  of  water. 

36.  Solubility.     Oxygen  is  slightly  soluble  in  water.     At 
ordinary  temperature  and  pressure  about  three  volumes  of 
oxygen  dissolve  in  one  hundred  volumes  of  water.     Thus 
oxygen  is  available  to  fishes,  for  it  can  pass  from  solution  in 
water  into  the  blood  of  the  fishes  through  the  delicate  mem- 


LIQUID  OXYGEN  43 

branes  of  the  gills,  —  the  gills  serving  fishes  in  the  way  that 
the  lungs  serve  human  beings. 

As  is  the  case  with  all  other  gases,  oxygen  is  more  soluble 
in  cold  water  than  in  warm  water.  It  is  a  frequently  ob- 
served fact  that  when  a  glass  of  freshly  drawn  cold  water 
is  allowed  to  stand  in  a  warm  room  little  bubbles  form  and 
cling  to  the  side  of  the  glass.  These  bubbles  are  composed  in 
part  of  oxygen.  Besides  being  essential  to  the  life  of  fishes,  the 
oxygen  dissolved  in  wrater  plays  a  large  part  in  purifying 
natural  waters  from  sewage  and  other  contamination.  Many 
of  the  harmful  substances  contained  in  sewage  combine  with 
oxygen  and  are  thus  converted  into  harmless  compounds. 

37.  Liquid   and   Solid   Oxygen.     We   are   familiar   with 
oxygen  only  as  a  gas,  but  to  the  scientist  with  the  requisite 
apparatus,  liquid  and  solid  oxygen  are  familiar  forms,  just 
as  ice  is  to  the  everyday  man  a  familiar  form  of  water.     If 
oxygen  gas  is  kept  in  a  vessel  at  the  ordinary  pressure  and 
cooled  sufficiently,  it  will  condense  to  a  liquid  when  the  tem- 
perature of  —  182.5°  C.  is  reached.     This  same  temperature, 
—  182.5°  C.,  is  also  the  boiling  point,  because  if  liquid  oxygen 
is  allowed  to  stand  in  an  open  vessel  it  boils  at  this  tempera- 
ture, that  is,  bubbles  of  gas  form  within  the  body  of  the 
liquid,  rise  to  the  surface,  and  escape.     In  the  same  way 
water  boils  at  100°  C. 

The  liquid  oxygen  has  a  blue  color,  it  is  somewhat  heavier 
than  water,  its  specific  gravity  being  l.lS,  and  it  shows  the 
surprising  property  of  being  magnetic. 

On  cooling  liquid  oxygen  still  further,  it  changes  to  a  solid 
at  about  —  200°  C.  in  the  same  way  that  water  changes  to 
ice  at  0°  C. 

38.  Chemical   Properties.     The   most   marked   chemical 
property  of  oxygen  is  its  great  combining  activity.     Com- 


44  OXYGEN 

pounds  of  it  with  every  other  element,  except  fluorine  and 
certain  inert  gases  of  the  atmosphere,  are  known.  More- 
over, it  will  combine  with  most  of  the  elements  directly,  par- 
ticularly when  heated  with  them.  This  was  seen  in  Chapter 
IV  to  be  the  case  with  many  metallic  elements,  and  it  is 
likewise  true  of  many  non-metallic  elements  such  as  phos- 
phorus, sulphur,  and  carbon. 

39.  Oxides.     The  product  formed  by  the  union  of  oxygen 
with  another  element  is  called  an  oxide.     It  is  a  custom 
among  chemists  in  naming  compounds  consisting  of  only 
two  elements  to  use  the  name  of  the  more  metallic  element 
in  full  and  the  root  of  the  name  of  the  more  non-metallic 
element  together  with  the  termination  ide.     Thus  sulphur 
and  oxygen  form  sulphur  oxide ;   iron  and  oxygen  form  iron 
oxide. 

40.  Combustion    in    Pure    Oxygen.     Fuels    when    once 
kindled  react  vigorously  with  the  oxygen  of  the  air  giving 
heat  and  light;    many  metals  when  wet  rust  or  corrode 
slowly,  which  is  in  the  main  a  combining  with  oxygen.     The 
power  to  support  active  combustion  is  much  stronger  with 
pure  oxygen  than  with  air.     For  example,  a  lump  of  charcoal 
with   a   barely  glowing    spark  flames   up   brilliantly   when 
thrust  into  a  jar  of  pure  oxygen.     Burning  bits  of  sulphur  or 
phosphorus  burn  far  more  vigorously  in  pure  oxygen.     Even 
a  piece  of  iron  wire,  if  a  glowing  bit  of  wood  is  attached  to 
its  end,  will    burn   vigorously  and    throw  off    a   beautiful 
shower  of  sparks,  when  lowered  into  pure  oxygen. 

41.  Kindling    Point.     At    ordinary    temperatures,    most 
combustible   materials   do   not   burn,  —  not   even   in   pure 
oxygen.     If  they  are  dry,  most  materials  can  be  preserved 
almost  indefinitely  in  oxygen  without  a  perceptible  amount 
of  chemical  union.     Thus  dry,  cold  oxygen  is  pretty  nearly 


KINDLING   POINT  45 

an  inert  substance  chemically.  It  is  probably  not  ab- 
solutely inert,  however,  for  there  is  perhaps  a  slow  com- 
bustion always  taking  place,  even  although  not  enough 
change  may  occur  in  one  hundred  or  one  thousand  years 
to  be  noticeable. 

Now  it  is  a  general  rule  that  chemical  reactions  take  place 
faster  the  higher  the  temperature.  Let  us  apply  this  rule, 
for  example,  to  a  piece  of  dry  wood  which  at  ordinary  tem- 
perature does  not  combine  perceptibly  with  the  oxygen  of 
the  air ;  if  the  wood  is  placed  in  a  cooking  oven,  it  combines 
without  doubt  far  more  rapidly,  but  the  reaction  is  still  too 
slow  to  produce  more  heat  on  its  own  account  than  can  easily 
escape.  Placed  in  hotter  and  hotter  places,  the  wood  reacts 
more  and  more  rapidly,  until  at  length  a  point  is  reached 
at  which  the  heat  is  produced  more  rapidly  by  the  reaction 
than  it  can  escape  from  the  vicinity  of  the  wood.  At  this 
point  the  wood  bursts  into  flame,  for  since  the  heat  cannot 
all  escape  it  must  raise  the  temperature  about  the  wood  still 
higher  and  this  higher  temperature  must  produce  a  more 
rapid  reaction  which  must  in  turn  raise  the  temperature  still 
higher. 

The  kindling  temperature  is  that  temperature  at  which  a  slow 
combustion  changes  abruptly  to  a  rapid  one  and  a  correspond- 
ing sudden  rise  in  temperature  occurs. 

The  kindling  temperature  of  different  substances  is  very 
different.  Thus  yellow  phosphorus  catches  fire  when  exposed 
to  air  even  at  the  ordinary  room  temperature,  sulphur  has  to 
be  heated  moderately,  whereas  charcoal  must  be  raised  to  a 
red  heat  before  it  will  burn. 

42.  We  can  now  appreciate  why  an  iron  wire  burns  bril- 
liantly in  pure  oxygen,  but  not  at  all  in  air.  The  kindling 
temperature  of  iron  is  high.  When  once  it  is  reached  in  pure 


46  OXYGEN 

oxygen,  the  heat  of  the  chemical  action  is  enough  to  maintain 
the  temperature  above  this  point.  But  in  air  there  are  four 
volumes  of  inert  nitrogen  to  each  volume  of  oxygen,  and  the 
nitrogen  as  well  as  the  oxygen  has  to  be  raised  to  the  kindling 
temperature  of  the  iron  if  the  iron  is  to  continue  to  burn. 
In  addition  to  this  the  oxygen  is  impeded  by  the  nitrogen 
in  reaching  the  surface  of  the  iron. 

In  extinguishing  a  fire  with  water,  the  main  function  of 
the  water  is  to  cool  the  burning  objects  to  below  the  kindling 
temperature. 

43.  Spontaneous   Combustion.     In   some   cases   of   slow 
oxidation  the  amount  of  heat  given  off,  although  not  enough 
to  cause  the  kindling  of  the  substance  concerned  when  it  is 
freely  exposed  to  the  air,  is  yet  sufficient  to  set  fire  to  it,  if  in  a 
situation  from  which  it  is  difficult  for  heat  to  escape.     Oily 
rags  sometimes  cause  fires  in  this  way,  if  thrown  into  corners. 
Some  kinds  of  oil  oxidize  quite  perceptibly  even  at  ordinary 
temperatures.     When  the  rags  are  packed  together  so  that 
the  heat  produced  by  this  slow  combustion  cannot  escape, 
the  temperature  rises  until  the  mass  breaks  into  flame.     This 
type  of  action  is  called  spontaneous  combustion,  and  it  is  a 
very  frequent  cause  of  factory  fires. 

44.  Slow  Combustion.     Dry   oxygen  is   almost  inert  at 
ordinary  temperatures,  but  dissolved  in  water  it  is  capable 
of  reacting  slowly,  as  is  shown  by  the  rusting  of  moist  iron. 
In  the  human  body  the  oxygen  dissolved  in  the  blood  com- 
bines with  the  elements  of  the  food,  giving  the  warmth  and 
energy  necessary  to  the  human  being.     This  is  a  wonderfully 
regulated  slow  combustion,  for  in  normal  health  it  is  never 
allowed  to  carry  the  temperature  much  above  the  normal 
body  temperature  of  37°  C.  nor  to  let  it  get  much  below  that 
mark  in  cold  weather. 


DECAY  OF  WOOD  47 

45.  Decay  of  Wood.     Dissolved  oxygen  is  also  responsible 
for  the  decaying  of  wood,  which  is  also  a  case  of  slow  combus- 
tion.    We  often  see  perfectly  sound  timbers  in  houses  over  a 
century  old  where  the  rain  has  been  carefully  kept  out,  but 
the  same  timbers  thrown  out  on  the  moist  ground  are  rotted 
through  in  a  year  or  two.     Moisture,  however,  is  not  all  that 
aids  in  the  decay  of  the  wood ;   an  important  part  is  played 
by    bacteria,    microscopic   living   organisms,  which   in    the 
chemistry  of  their  life  processes  bring  about  the  oxidation 
of    the   wood   in   much   the   same   way   as   animals   bring 
about  the  oxidation  of  their  food.     Wood  that  is  to  be  used 
in   wet  places,  as  for  example,  piles  for  buildings,  is  pre- 
served by  being  saturated  with  creosote  or  other  material 
poisonous  to  bacteria. 

Dry  wood  does  not  decay  because  the  bacteria  can  live 
only  when  wet  or  at  least  moist.  Painting  preserves  wood 
because  it  excludes  both  moisture  and  oxygen. 

46.  The  spoiling  of  food  is  similar  to  the  decay  of  wood 
in  that  it  is  due  to  chemical  changes  which  are  promoted  by 
bacteria,  although  many  of  these  changes  consist  only  of  the 
partial  decomposition  of  the  complex  compounds  of  which 
food  consists,  and  do  not  involve  any  action  of  the  oxygen 
of  the  air.     Thus  the  souring  of  milk  is  due  to  a  decomposi- 
tion of  milk  sugar  into  lactic  acid  under  the  influence  of  a 
certain  kind  of  bacteria.     The  fruit  sugar  of  apples  decom- 
poses into  carbon  dioxide^and  alcohol  under  the  influence  of 
yeast  cells  when  sweet  cider  ferments.     When  cider  or  wine 
turn  to  vinegar  it  is  due  to  the  oxidation  of  alcohol  to  acetic 
acid  through  the  agency  of  the  acetic  bacteria.     This  change 
is  an  oxidation  and  it  can  only  progress  when  the  cider  or 
wine  is  exposed  to  the  air.     The  well  known  "mother  of 
vinegar  "  which  is  often  seen  in  vinegar  bottles  consists  of  a 

B.  AND  W.  CHEM. 4 


48  OXYGEN 

cohering  mass  of  the  acetic  bacteria.  When  meats  and 
fish  spoil  the  products  are  very  poisonous  and  this  kind  of 
spoiling  may  take  place  in  the  absence  of  oxygen. 

Anything  which  will  kill  bacterial  life  will  stop  the  spoil- 
ing of  food.  Thus  to  make  milk  keep  longer  without  sour- 
ing, we  heat  it.  In  canning  fruit  or  meat,  the  material  is 
heated  as  high  as  the  boiling  temperature  of  water  in  order 
to  kill  bacteria.  In  the  packing  houses,  the  cans  after  sealing 
are  invariably  heated  again  to  assure  the  destruction  of  all 
bacterial  life. 

SUMMARY 

Oxygen  is  prepared  in  bulk,  most  cheaply  by  methods  by  which 
it  is  extracted  from  the  air,  but  for  the  laboratory  the  most 
convenient  method  is  to  heat  potassium  chlorate,  to  which 
some  manganese  dioxide  has  been  added. 

Properties  of  Oxygen.  Oxygen  can  generally  be  recognized  by  its 
easily  observed  properties.  It  is  a  colorless,  odorless  gas, 
slightly  soluble  in  water.  The  most  marked  chemical  char- 
acteristic of  oxygen  is  its  great  power  to  combine  with  other 
elements.  Oxygen  under  standard  conditions  weighs  1.43 
grams  per  liter. 

The  kindling  temperature  is  the  lowest  temperature  at  which  slow 
combustion  changes  to  rapid  combustion.  The  kindling 
point  is  different  for  different  substances.  Above  the  kindling 
temperature,  oxygen  combines  violently  with  many  substances. 

Slow  combustion  consists  in  the  union  of  a  substance  with  oxygen, 
below  its  kindling  temperature,  as  in  the  rusting  of  iron,  the 
decaying  of  wood,  and  the  spoiling  of  food.  The  latter  two 
actions  are  promoted  by  bacterial  life. 

Spontaneous  combustion  results  when,  during  slow  combustion, 
heat  accumulates  until  the  kindling  temperature  is  reached, 
then  the  substance  takes  fire. 

Questions 

1.  What  is  the  most  prominent  chemical  property  of  oxygen? 

2.  Why  do  metals,  such  as  copper,  lead,  and  zinc,  which  oxidize 


QUESTIONS  49 

,   l      • 

quickly  when  hot,  wear  as  well  as  they  do  when  exposed  to  the  air 
on  roofs  ? 

3.  Why  does  not  gold  corrode? 

4.  What  would  be  the  effect  of  using  pure  oxygen  in  the  black- 
smith's bellows? 

5.  Give  reasons  why  it  is  fortunate  that  the  atmosphere  does 
not  consist  entirely  of  oxygen. 

6.  In  view  of  the  combustibility  of  wood,  why  is  it  that  the  fire 
danger  in  wooden  buildings  is  so  slight? 

7.  Under  what  conditions  do  fires  sometimes  start  without  the 
application  of  a  flame? 

8.  Would  oxygen  be  a  useful  gas  with  which  to  fill  balloons? 
Explain. 

9.  How  do  fish  breathe?     What  would  be  the  effect  of  putting 
goldfish  in  distilled  water?     Why  would  freshly  distilled  water  lack 
oxygen? 

10.  Explain  why  a  fallen  tree  in  the  forest  disappears  after  a 
period  of  years. 

11.  It  is  unsafe  to  drink  water  from  a  river  that  has  been  freshly 
contaminated  with  sewage.     How  is  oxygen  responsible  for  making 
the  water  safe  to  drink  after  it  has  run  for  miles  over  rapids  in  a 
river  ? 

12.  How  does  (a)  canning,  (b)  smoking,  (c)  salting,  (d)  refrigerat- 
ing, (e)  drying,  and  (/)  preserving  in  sirup,  prevent  in  food  products 
the  kind  of  slow  oxidation  called  decay? 


CHAPTER  VI 
THE    OXIDES    OF    CARBON 

IN  the  preceding  chapters  it  has  been  seen  that  when  the 
element  carbon  burns,  it  unites  with  oxygen  of  the  air  to 
form  a  gaseous  product  which  is  always  carbon  dioxide  when 
sufficient  air  can  come  in  contact  with  the  burning  object, 
although  carbon  monoxide  may  result  with  insufficient  air. 

47.  Enormous  Production  of  Carbon  Dioxide.     Enormous 
quantities  of  charcoal,  coke,  coal,  and  wood,  —  fuels  which 
consist  largely  or  wholly  of  carbon,  are  being  burned  all  the 
time.     Added  to  this  artificial  combustion,  we  have  an  even 
greater  production  of  carbon  dioxide  by  the  slow  combustion 
of  food  in  human  beings  and  animals  and  of  wood  and  dead 
leaves  in  the  process  of  decay.     It  has,  in  fact,  been  estimated 
that  97  per  cent  of  the  carbon  dioxide  turned  into  the  air 
is  the  product  of  decay. 

48.  Utilization  of  Carbon  Dioxide  by  Plants.     In  view  of 
the  great  amount  of  carbon  dioxide  produced,  it  would  seem 
as  if  the  air  must  become  filled  with  this  gas.     Still  it  is  a 
fact  that  pure  air  in  the  country  never  contains  more  than 
three  or  four  one  hundredths  of  1  per  cent  of  it.     How  can 
it  be  that  such  large  quantities  of  this  gas  are  all  the  time 
being  put  into  the  air  and  yet  the  amount  present  never 
increases  ?     It  must  be  because  carbon  dioxide  is  being  con- 
tinually withdrawn  from  the  air.     The  study  of  the  life  of 
green  plants  has  shown  that  it  is  they  that  perform  this 

60 


LIMESTONE  51 

service;  they  require  carbon  dioxide  for  their  development, 
and  their  green  leaves  take  in  the  carbon  dioxide  from  the 
air.  But  this  is  not  all.  The  lungs  of  animals  withdraw 
oxygen  from  the  air  and  give  back  carbon  dioxide ;  the  green 
leaves  of  plants  withdraw  the  carbon  dioxide  and  after  ap- 
propriating the  carbon  in  their  chemical  life  processes,  give 
back  the  oxygen.  Thus  there  is  a  continuous  interchange 
between  animals  and  plants,  each  utilizing  what  is  a  waste 
product  from  the  other,  and  with  these  two  agencies  at  work 
the  quantity  of  carbon  dioxide  in  the  air  never  varies  much 
from  the  mean  of  0.04  per  cent. 

49.  Source  of  Pure  Carbon  Dioxide.     In  our  laboratory 
study  of  carbon  dioxide  our  first  task  is  to  collect  some  of 
this  gas  pure.     One  might  first  think  to  do  this  by  collecting 
the  gaseous  combustion  products  of  charcoal,  but  it  would 
prove  difficult  to  get  the  gas  in  this  way  unmixed  with  nitro- 
gen and  unconsumed  air.     Our  best  source  of  carbon  dioxide 
has  been  found  to  be  limestone,  a  rock  that  is  very  abundant 
in  the  earth. 

50.  Limestone.     In  past  geologic  ages,  plants  have  been 
absorbing  carbon  dioxide  from  the  air,  just  as  at  present. 
Certain  low  orders  of  sea  animals  also  have  been  utilizing 
carbon  dioxide  in  the  formation  of  their  shells,  in  which  it 
becomes  combined  and  held  in  the  form  of  calcium  carbonate. 
Coral,  for  example,  and  the  shells  of  oysters  and  clams  con- 
sist largely  of  calcium  carbonate.     Masses  of  these  and  simi- 
lar shells  which  accumulated  in  past  ages,  have  been  deeply 
covered  by  deposits  of  earth.     They  have  since  been  trans- 
formed gradually,  through  the  agency  of  great  pressure  and 
heat,  into  compact  masses  of  limestone. 

A  very  pure  variety  of  limestone  is  marble,  and  in  the 
laboratory  we  shall  use  marble  as  the  source  of  carbon  dioxide. 


52 


THE   OXIDES  OF  CARBON 


FIG.  7. — Test  for  Carbon  Dioxide.  A  clear 
drop  of  limewater  suspended  on  a  stirring  rod 
is  rendered  milky  if  lowered  into  a  vessel  con- 
taining carbon  dioxide.  Notice  that  the  stopper 
of  the  limewater  bottle  is  held  between  the 
third  and  fourth  ringers  at  the  back  of  the  right 
hand  instead  of  being  laid  on  the  desk  top 
where  it  may  be  contaminated  before  being 
replaced  in  bottle. 


It  is  only  necessary 
to  treat  this  sub- 
stance with  a  dilute 
acid  when  carbon 
dioxide  is  liberated, 
while  the  product 
formed  from  the  cal- 
cium and  the  acid 
remains  behind. 

51.  Means  of  rec- 
ognizing Carbon 
Dioxide.  If  a  pinch 
of  powdered  marble 
is  placed  in  a  test  tube 
and  covered  with  a 
few  cubic  centi- 
meters of  dilute  acid, 
—  hydrochloric  acid, 
for  example,  —  a 
vigorous  efferves- 
cence 1  takes  place 
and  continues  until 
either  the  marble  or 
the  acid  is  all  used 
up. 

If  a  lighted  taper 
is  inserted  into  the 


test  tube  while  the  effervescence  is  taking  place,  the  flame  is 

1  Effervescence  consists  in  the  rapid  evolution  of  bubbles  of  a 
gas  and  in  this  respect  it  has  somewhat  the  appearance  of  boiling1. 
It  is,  however,  an  entirely  different  phenomenon  from  boiling,  since 
in  boiling  the  vapor  that  rises  is  always  the  same  substance  as  the 
liquid  from  which  it  comes. 


CARBON   DIOXIDE  53 

extinguished.  This  accords  with  our  knowledge  that  carbon 
dioxide  is  a  non-supporter  of  combustion.  If  a  stirring  rod 
is  dipped  into  a  bottle  of  clear  limewater  and  then  with- 
drawn so  that  a  clear  drop  hangs  from  its  lower  end,  and  if 
this  is  then  lowered  into  the  test  tube,  it  immediately 
becomes  clouded  or  milky  in  appearance.  This  test  has 
already  been  mentioned  and  is  one  of  the  most  distinctive 
tests  for  carbon  dioxide. 

If  the  last  two  tests  are  made  repeatedly  in  inverted  and 
upright  test  tubes  in  which  carbon  dioxide  has  been  gener- 
ated, it  will  be  found  that  all  evidence  of  the  gas  disappears 
in  a  moment  from  an  inverted  test  tube,  whereas  it  persists 
for  some  time  in  an  upright  test  tube.  This  shows  that 
carbon  dioxide  must  be  heavier  than  air. 

The  three  properties  of  carbon  dioxide  enumerated  in  the 
last  few  paragraphs,  namely  its  power  to  extinguish  flame, 
its  ability  to  render  limewater  milky,  and  its  being  heavier 
than  air,  are  all  properties  which  can  be  recognized  with 
great  ease  and  it  is  these  properties  for  which  we  first  look 
when  we  wish  to  find  out  whether  a  given  gas  is  or  is  not 
carbon  dioxide.  If  we  find  the  gas  to  possess  these  three 
properties,  we  are  reasonably  certain  that  it  is  carbon  dioxide. 

Of  course  to  render  identification  of  this  gas  absolutely 
certain,  its  properties  must  be  observed  with  much  greater 
exactness  and  to  accomplish  this  it  is  necessary  to  prepare 
some  considerable  quantity  of  the  gas 'and  to  collect  it 
unmixed  with  air. 

52.  Carbon  Dioxide  Generator.  Place  several  lumps  of 
marble  in  the  bottom  of  a  bottle  or  flask  (see  Fig.  8)  fitted 
with  a  two-holed  rubber  stopper.  Through  one  hole  is 
inserted  a  thistle  tube  which  reaches  quite  to  the  bottom  of 
the  generating  flask,  where  it  is  sealed  air-  or  gas-tight  when 


54 


THE  OXIDES  OF  CARBON 


a  little  liquid  is  poured  into  the  flask.  Through  the  other 
hole  of  the  stopper  passes  a  delivery  tube  and  this  tube  is 
prolonged  so  that  it  reaches  below  the  surface  of  the  water 
in  the  trough  where  are  placed  inverted  bottles  filled  with 
water  ready  for  collecting  the  gas. 

It  will  be  recollected  that  this  method  of  displacing  water 
from  inverted  bottles  immersed  in  a  trough  of  water  was 

also  used  for  collect- 
ing oxygen.  This  is 
in  fact  the  usual 
method  of  collecting 
gases,  and  may  be 
used  for  any  gas  that 
is  not  very  soluble  in 
water. 

Now  with  the  gen- 
erator   and     connec- 
Cracked    tions   all   arranged  a 
Enough    little  water  is  poured 


FIG.  8.  —  Carbon  Dioxide  Generator, 
marble  is  placed  in  generator  flask, 
water  is  added  to  close  the  lower  end  of  the 
thistle  tube.     Then  hydrochloric  acid  is  added 
a  little  at  a  time  to  obtain  a  moderate  flow  of 
the  gas. 


into  the  thistle  tube 
until  its  lower  end 
within  the  generator 

is  submerged  and  thus  sealed  so  that  no  gas  can  escape 
this  way.  Then  concentrated  hydrochloric  acid  is  added, 
a  few  drops  at  a  time,  until  a  fairly  brisk  effervescence 
takes  place  in  the  flask. 

The  first  gas  to  issue  from  the  delivery  tube  should  be 
rejected  because  it  is  mixed  with  the  air  that  was  originally 
in  the  generator,  but  after  considerable  gas  has  escaped,  the 
air  will  all  have  been  swept  from  the  generator  and  pure 
carbon  dioxide  can  be  collected  by  sliding  the  in  verted  bottles 
of  water  over  the  end  of  the  delivery  tube. 


CARBON  DIOXIDE 


55 


PROPERTIES  OF  CARBON  DIOXIDE 

53.  Specific  Gravity.     Accurate  experiments  have  shown 
that  carbon  dioxide  weighs  1.529  times  as  much  as  air,  in 
other  words  that  its  specific  gravity  is  1.529.     It  is  not 
necessary  or  even  desirable  for  every  student  of  chemistry  to 
repeat   the  somewhat 

elaborate  experiment 
necessary  to  prove 
the  correctness  of  this 
exact  figure,  which 
has  been  obtained  by 
skillful  and  reliable 
experimenters. 

A  striking  experi- 
ment for  showing  the 
greater  density  of  car- 
bon dioxide  is  illus- 
trated in  Fig.  9.  Sev- 
eral lighted  candles 
are  placed  in  an  in- 
clined trough ;  then 
carbon  dioxide,  pre- 
viously collected  in 
a  large  cylinder,  is 
poured  into  the  top  of  the  trough.  It  is  shown  by  the 
candles  becoming  successively  extinguished  that  the  carbon 
dioxide  flows  down  hill  through  the  trough. 

54.  Solubility  in  Water.     Carbon  dioxide  is  not  very  solu- 
ble in  water,  as  has  been  shown  by  the  fact  that  it  has  been 
collected  in  bottles  inverted  over  water.     It  could  be  shown, 
however,  that  some  of  the  gas  dissolved  in  the  water  through 


FIG.  9.  —  Carbon  Dioxide  flowing  down  hill. 


56  THE  OXIDES  OF  CARBON 

which  it  bubbled.  Close  the  mouth  of  one  of  the  bottles  of 
carbon  dioxide  inverted  in  the  collecting  trough  by  placing 
the  palm  of  the  hand  under  it,  transfer  the  bottle  to  another 
trough  containing  fresh  cold  water.  On  letting  the  bottle 
stand  in  this  trough  a  few  hours,  it  is  found  that  the  water 
level  within  the  bottle  rises  very  considerably,  showing  that 
some  of  the  gas  has  disappeared  and  the  water  risen  to  take 
its  place.  The  only  way  in  which  the  gas  can  disappear  is 
by  dissolving  in  the  water. 

At  ordinary  temperature  and  with  the  gas  under  the  pres- 
sure of  the  atmosphere,  a  volume  of  water  is  capable  of  dis- 
solving about  its  own  volume  of  carbon  dioxide  gas.  Under 
less  pressure,  less,  and  under  greater  pressure,  more,  gas  can 
be  dissolved.  Plain  soda  water,  more  correctly  called  car- 
bonated water,  obtained  at  a  soda  fountain  or  dispensed 
in  siphon  bottles,  is  simply  a  solution  of  carbon  dioxide  in 
water.  The  carbon  dioxide  is  forced  under  a  high  pressure 
into  bottles  or  metal  tanks  of  water,  and  under  the  high  pres- 
sure a  correspondingly  large  amount  dissolves.  When  the 
carbonated  water  is  drawn  into  a  glass,  the  pressure  is  re- 
lieved, and  we  see  most  of  the  carbon  dioxide  escaping  in  the 
bubbles  which  rapidly  rise. 

55.  Liquid  Carbon  Dioxide.  Dry  carbon  dioxide  gas 
may  be  liquefied  by  compression  at  the  ordinary  room  tem- 
perature. At  20°  C.  a  pressure  of  861  pounds  per  square 
inch  or,  in  other  words,  a  pressure  59  times  as  great  as  that 
of  the  atmosphere,  is  required.  Of  course  very  strong  con- 
tainers are  necessary  to  withstand  this  pressure,  and  it  is 
customary  to  compress  the  gas  into  steel  cylinders  in  which 
it  is  put  on  the  market.  Compression  always  produces  heat, 
and  unless  the  compressing  apparatus  and  the  receiving 
cylinder  are  surrounded  by  cold  running  water  to  carry  away 


SOLID   CARBON   DIOXIDE  57 

the  heat,  the  temperature  will  rise  and  liquefaction  will  be 
impossible. 

56.  Solid  Carbon  Dioxide.  If  the  cylinder  of  liquid 
carbon  dioxide  is  placed  with  the  valve  at  the  bottom  and 
the  valve  is  opened,  some  of  the  liquid  will  be  forced  out. 
But  here  we  make  the  surprising  observation  that  we  do  not 
obtain  any  liquid  at  all  outside  of  the  pressure  cylinder,  but 
we  do  get  a  solid  substance  which  looks  like  snow.  A  good 
way  to  obtain  this  snow  is  to  tie  securely  a  stout,  coarse  bag 
over  the  escape  pipe  of  the  cylinder  and  open  the  valve  for 
a  few  minutes.  Considerable  frost  can  be  seen  on  the  out- 
side of  the  bag  and  the  hissing  of  escaping  gas  is  heard.  On 
closing  the  valve  and  opening  the  bag,  several  handfuls  of 
carbon  dioxide  snow  are  found. 

On  compressing  carbon  dioxide  gas  to  a  liquid,  we  just 
said  that  a  large  amount  of  heat  is  produced  that  has  to  be 
removed  with  cold  water.  Now  on  letting  liquid  carbon 
dioxide  escape  from  the  cylinder,  an  equal  amount  of  heat 
must  be  supplied  from  somewhere  if  the  liquid  is  to  be  changed 
back  to  a  gas.  Enough  heat  is  not  at  once  available  from  the 
liquid  and  its  immediate  surroundings,  but  what  there  is,  is 
taken,  and  some  of  the  liquid  is  thereby  changed  to  gas. 
This  withdrawal  of  heat  produces  great  cold  and  it  is  thus 
that  the  remaining  carbon  dioxide  is  frozen.  The  tem- 
perature of  the  snow  is  as  low  as  —60°  C.  It  could  serve  thus 
as  an  excellent  refrigerating  agent  for  temperatures  below 
that  of  the  ordinary  ice-salt  mixture. 

This  carbon  dioxide  snow  is  a  fascinating  substance  to 
handle.  Holding  a  little  lump  in  the  palm  of  the  hand,  it 
may  be  seen  to  evaporate.  A  lump  thrown  into  a  dish  of 
water  causes  the  water  to  freeze.  Mercury  poured  upon  it 
at  once  becomes  solid.  In  handling  it,  however,  one  must 


58  THE  OXIDES  OF  CARBON 

avoid  pressing  it  against  the  flesh,  else  it  causes  frostbite, 
the  effects  of  which  are  quite  similar  to  those  of  burning. 

57.  Chemical  Properties  of  Carbon  Dioxide.     We  have 
already  seen  that  carbon  dioxide  is  absorbed  by  limewater 
and  produces  in  it  a  white  turbidity,  also  that  it  dissolves 
to  some  extent  in  water. 

We  know  that  a  glass  of  plain  soda  water,  —  that  is,  the 
carbonated  water  without  the  addition  of  fruit  sirups, — 
tastes  somewhat  sour.  Carbon  dioxide,  in  fact,  forms  a 
weak  acid  with  the  water  in  which  it  dissolves.  We  shall 
learn  more  later  about  acids,  but  it  is  true  that  all  acids  are 
sour  in  taste.  Another  general  property  of  acids  is  that  they 
cause  the  color  of  a  vegetable  dyestuff  called  litmus  to  change 
from  blue  to  red.  When  a  piece  of  paper  colored  blue  with 
litmus  is  dipped  into  water  containing  dissolved  carbon 
dioxide,  the  color  is  changed  to  a  rather  pale  red. 

Another  property  ofjicids  is  that  they  react  with  the  oxide^ 
of  metals  to  form  salts.  Now  limewater  is  obtained  by  allow- 
ing lime,  which  is  oxide  of  calcium,  to  react  with  water. 
The  product  of  the  reaction  is  slightly^  soluble  in  water  and 
the  clear  solution  is  known  as  limewater.  Carbon  dioxide 
reacting  with  the  limewater  gives  calcium  carbonate,  a  white 
insoluble  salt  which  produces  the  milky  appearance  which 
we  have  described  as  a  distinctive  test  for  carbon  dioxide. 

USES  OF  CARBON  DIOXIDE 

58.  Fire  Extinguishers.     The  experiment  of  extinguish- 
ing the  burning  candles  suggests  one  of  the  important  uses 
of  carbon  dioxide,  namely,  as  a  fire  extinguisher.     The  so- 
called  automatic  fire  extinguishers  are  a  familiar  sight  in 
the  corridors  of  public  buildings  and  in  railway  cars.     The 
most  useful  form  consists  merely  of  a  device  for  manufactur- 


FIRE  EXTINGUISHERS 


59 


ing,  when  the  occasion  may  suddenly  demand  it,  a  large 
amount  of  carbon  dioxide  which  will  charge  the  water  in  the 
extinguisher  and  at  the  same  time  create 
a  pressure  to  force  the  charged  water 
through  the  nozzle  so  that  it  can  be  di- 
rected at  the  fire.  The  carbonated  water, 
on  being  relieved  from  the  great  pressure 
and  moreover  warmed  on  striking  the  fire, 
gives  up  a  large  amount  of  the  carbon 
dioxide,  which  it  can  no  longer  hold  in 
solution.  The  gas,  being  heavier  than 
air,  tends  to  spread  over  the  fire  and 
exclude  the  air.  The  water  thrown  by 
the  extinguisher  also  helps  to  put  out  the 
fire  by  cooling  the  fuel  below  its  kindling 
point,  but  the  efficiency  of  one  of  these 
so-called  chemical  fire  extinguishers  is 
greater  than  if  only  the  same  amount  of 
water  were  thrown. 

The  extinguisher  is  usually  constructed 
as  shown  in  the  diagram  (see  Fig.  11). 
The  metal  tank  is  filled  with  water  nearly  to  the  top 
(the  level  of  the  water  is  seen  at  A)  and  a  proper  quantity 
of  some  carbonate,  usually  sodium  bicarbonate,  or  com- 
mon baking  soda,  is  added  and  allowed  to  dissolve  in 
the  water.  Marble  would  be  too  slow 'in  its  action,  but 
the  soluble  carbonate  being  infinitely  more  finely  divided 
when  in  the  dissolved  state  than  the  most  finely  ground 
marble  dust,  is  much  more  rapidly  acted  upon.  The 
amount  to  add  would  be  determined  by  the  capacity  and 
strength  of  the  tank.  Directions  always  accompany  the 
chemical  extinguishers  and  state  how  much  sodium  bicar- 


FIG.  10.  —  Chemical 
Fire  Extinguisher. 


60 


THE   OXIDES  OF   CARBON 


bonate  and  sulphuric  acid  to  use.  In  the  top  of  the  tank  is 
a  support  for  a  bottle  of  concentrated  sulphuric  acid.  This 
support  (B)  is  made  of  some  metal  little 
likely  to  corrode  in  a  damp  place.  The 
stopper  of  the  sulphuric  acid  bottle  (C) 
is  of  lead  and  fits  loosely.  The  hose 
through  which  the  contents  of  the  tank 
are  to  be  discharged  is  attached  to  what 
is  the  top  of  the  tank  when  the  latter 
is  not  in  use.  When  occasion  arises  to 
use  the  extinguisher,  it  is  taken  to  the 
scene  of  the  fire  and  at  once  inverted. 
The  sulphuric  acid  is  thus  emptied  out 
into  the  solution  of  sodium  bicarbonate, 
and  rapid  evolution  of  carbon  dioxide 
begins.  The  pressure  rises  and  the  so- 
lution is  expelled  forcibly  through  the 
hose  which  leads  from  what  is  now  the 
bottom  of  the  tank.  If  the  hose  were 
not  at  the  bottom  (when  in  action),  of 
course  only  gas  would  escape. 
59.  Soda  Water.  The  use  of  carbon  dioxide  dissolved 
in  water  in  the  production  of  effervescing  drinks  has  already 
been  referred  to  in  the  paragraph  on  the  solubility  of  carbon 
dioxide.  The  original  soda  water  was  prepared  by  mixing' 
baking  soda  (sodium  bicarbonate)  in  solution  with  some 
fruit  acid.  Carbon  dioxide  was  produced  as  is  the  case 
when  any  acid  acts  on  any  carbonate,  and  the  water  was  thus 
charged  with  the  gas.  But  carbon  dioxide  is  only  one  of  the 
products  of  the  action  of  sodium  bicarbonate  with  an  acid. 
The  other  product  is  a  salt,  the  sodium  salt  of  the  fruit  acid. 
This  salt  is  not  harmful,  but  it  does  not  add  to  the  attrac- 


FIG.  11.— Section  of 
Fire  Extinguisher. 


RAISING   OF   BREAD   AND   CAKE  61 

tiveness  of  the  beverage.  To-day  soda  water  is  universally 
made  by  charging  pure  water  in  strong  metal  tanks  with 
pure  carbon  dioxide  gas.  Thus  the  modern  soda  water  really 
contains  no  soda  at  all. 

60.  Raising  of  Bread  and  Cake.  In  making  bread  and 
cake  it  is  essential  that  the  product  shall  be  light  and  porous, 
for  otherwise  it  would  be  neither  appetizing  to  eat  nor  easy 
to  digest.  Lightness  is  obtained  through  the  agency  of 
carbon  dioxide,  which  is  generated  throughout  the  mass  of 
the  dough,  either  by  the  action  of  yeast  or  of  baking  powder, 
so  that  the  Plough  becomes  filled  with  a  multitude  of  little 
bubbles.  These  bubbles  "  raise  "  the  bread. 

Yeast  consists  of  a  vast  number  of  little  plant  cells,  which 
grow  and  multiply  very  rapidly  when  put  in  the  moist  dough, 
and  in  the  process  of  their  growth  cause  a  chemical  change 
in  the  starch  of  the  flour  and  in  the  sugars  present  in 
the  dough.  The  final  products  of  this  change  are  chiefly 
alcohol  and  carbon  dioxide.  The  amount  of  alcohol  is  so 
small  that  it  is  quite  unnoticeable ;  but  the  carbon  dioxide, 
being  a  gas,  has  a  large  volume  in  proportion  to  its  weight 
and  thus  it  causes  the  expansion  of  the  bread. 

Baking  powder  is  made  by  mixing  sodium  bicarbonate 
with  some  dry  powdered  acid  substance,  as  for  example 
cream  of  tartar.  So  long  as  the  baking  powder  remains  dry 
no  action  occurs ;  but  when  it  is  moistened,  the  acid  reacts 
with  the  carbonate  and  produces  carbon  dioxide  gas. 

The  baking  powder  should^therefore  be  first  well  mixed 
with  the  flour  so  that  gas  will  be  released  equally  in  every 
part  of  the  mass  and  the  mixing  of  the  flour  with  the  watery 
materials  (milk,  water,  or  eggs)  should  take  place  just  before 
the  material  is  to  go  into  the  oven  so  as  to  avoid  the  loss  of  gas. 

Baking  soda,  which  is  sodium  bicarbonate,  is  frequently 


62  THE  OXIDES  OF  CARBON 

used  with  sour  milk  to  bring  about  the  raising  of  homemade 
products  such  as  griddle  cakes  or  gingerbread.  In  this  case, 
carbon  dioxide  is  released  from  the  sodium  bicarbonate  by 
the  action  of  lactic  acid  in  the  sour  milk.  Frequently  the 
housewife  takes  too  much,  or  perhaps  too  little,  soda  for  the 
amount  of  acid  in  the  sour  milk.  The  resulting  product 
may  then  taste  strongly  either  of  the  excess  of  soda  or  of  acid. 
In  making  baking  powder,  the  manufacturers  are  careful  to 
properly  proportion  the  two  ingredients  so  that  no  excess  of 
either  shall  be  left  unchanged  at  the  end  of  the  reaction. 

61.  Commercial  Methods   of  making    Carbon  Dioxide. 
Sulphuric  acid  is  cheaper  than  hydrochloric  acid,  and  it 
is  therefore  used  in  the  commercial  manufacture  in  preference 
to  the  more  expensive  acid  which  is  most  often  used  in  the 
laboratory.     With  the  strong  effervescence,  a  spray  carrying 
considerable  sulphuric  acid  is  thrown  up,  and  therefore  the 
gas  must  be  passed  through  a  washing  vessel  containing  pure 
water  and  best  also  strained  through  some  porous  material 
before  being  charged  into  the  siphons  of  carbonated  water 
or  liquefied  into  steel  cylinders. 

Carbon  dioxide  is  the  chief  gaseous  product  from  the  fer- 
mentation of  grain,  and  large  quantities  of  this  gas  are 
produced  in  distilleries  and  breweries.  Of  late  years  the 
practice  has  grown  not  to  allow  this  gas  to  go  to  waste,  but 
to  collect  it  and  compress  it  in  steel  cylinders,  so  that 
to-day  a  large  part  of  the  carbon  dioxide  in  use  is  obtained 
in  this  way. 

CARBON  MONOXIDE 

62.  Formation.     In  connection  with  our  study  of  combus- 
tion (page  31)  it  was  said  that  if  carbon  is  burned  in  an  in- 
sufficient supply  of  oxygen,  a  gas  called  carbon  monoxide  is 


CARBON  MONOXIDE  63 

produced.  The  most  favorable  conditions  for  the  formation  of 
carbon  monoxide  exist  in  a  deep  coal  or  charcoal  fire  to  which 
air  is  admitted  only  at  the  bottom.  The  air  at  the  bottom 
doubtless  burns  some  of  the  carbon  to  carbon  dioxide.  The 
heat  of  the  reaction  brings  the  layer  of  fuel  above  to  incan- 
descence, and  any  carbon  dioxide  starting  to  pass  up  through 
this  layer  of  white-hot  carbon  reacts  with  it  and  thus  becomes 
changed  back  to  carbon  monoxide. 

63.  Properties  of  Carbon  Monoxide.    Physical  Properties. 
Carbon  monoxide  is  colorless  and  odorless.     It  is  lighter 
than  carbon  dioxide,  being  a  trifle  less  heavy  than  air,  whereas 
carbon  dioxide,  as  we  have  seen,  is  about  one  and  one  half 
times  as  heavy  as  air.     It  is  very  slightly  soluble  in  water. 

Chemical  Properties.  Carbon  monoxide,  unlike  carbon 
dioxide,  does  not  react  withrwater  to  form  an  acid.  Its  most 
striking  and  useful  property  is  its  ability  to  take  on  more 
oxygen  when  burning,  thus  forming  carbon  dioxide.  Carbon 
monoxide  is  excessively  poisonous  to  breathe,  even  when 
largely  diluted  with  air.  Hence  the  danger  from  leaky  fur- 
naces and  from  leaking  water  gas  (see  below). 

64.  Uses  of  Carbon  Monoxide.     Large  amounts  of  carbon 
monoxide,  usually  mixed  with  other  fuel  gases,  are  prepared 
and  used  as  artificial  gas  for  -heating  and  lighting.     For 
lighting,  other  gases  must  be  present  as  carbon  monoxide 
itself  burns  with  a  non-luminous  flame.     Water  gas,  which 
is  a  variety  of  illuminating  gas  used  ver^  largely  in  cities, 
and  producer  gas  which  is  much  employed  as  a  fuel  gas,  both 
contain  very  high  proportions  of  carbon  monoxide. 

65.  Combining  Proportions.     Carbon  dioxide  and  carbon 
monoxide  are  both  composed  of  exactly  the  same  elements, 
yet  the  two  gases  have  very  different  properties,  as  different 
in  fact  as  two  gases  composed  of  totally  different  elements. 

B.  AND  W.  CHEM. 5 


64  THE  OXIDES  OF  CARBON 

The  difference  is  to  be  accounted  for  only  on  the  ground  of 
the  difference  in  the  proportion  of  the  two  elements  com- 
bined. 

In  carbon  monoxide  one  gram  of  carbon  is  combined  with 
1.333  grams  of  oxygen.  In  carbon  dioxide  one  gram  of 
carbon  is  combined  with  2.666  grams  of  oxygen.  The  re- 
markably simple  relation  is  at  once  perceived  —  that  for  a 
given  amount  of  carbon  (one  gram  in  each  of  the  above  cases) 
the  second  gas  contains  exactly  twice  as  much  oxygen  as  the 
first  gas.  Hence  the  name  di-oxide,  the  prefix  di  coming 
from  the  Greek  and  meaning  two.  In  distinction,  the  prefix 
mono  means  one. 

66.  Law  of  Multiple  Proportions.  Besides  carbon  and 
oxygen  there  are  many  pairs  of  elements  which  combine  in 
more  than  one  proportion.  We  have  found  the  very  remark- 
able fact  that  the  two  combining  ratios  of  carbon  and  oxygen 
bear  the  simple  relation  to  each  other  of  two  to  one.  Now  it 
has  been  found  that  in  all  cases  of  more  than  a  single  combin- 
ing ratio,  the  relations  are  of  the  same  simple  nature.  A  gen- 
eral statement  of  this  fact  is  known  as  the  Law  of  Multiple 
Proportions. 

We  shall  see  later  that  this  law,  together  with  the  law  of 
definite  proportions,  at  which  we  have  already  hinted 
(page  22),  are  of  the  greatest  significance,  since  they  point  the 
way  to  our  present  conception  of  the  nature  of  matter, 
namely,  to  the  theory  of  atoms. 

SUMMARY 

Carbon  Dioxide  in  Nature.  The  amount  of  carbon  dioxide  being 
continually  turned  into  the  atmosphere  by  fires,  the  exhala- 
tions of  animals,  and  decay  is  enormous,  yet  the  amount  present 
does  not  increase. 


QUESTIONS  65 

Green  plants  absorb  carbon  dioxide,  using  the  carbon  and  re- 
turning oxygen  to  the  air. 

Large  amounts  of  limestone,  a  rock  which  contains  carbon  dioxide 
united  with  the  oxide  of  calcium,  are  found  in  nature.  Some 
mountain  ranges  are  composed  mainly  of  this  one  kind  of 
rock.  Marble  is  a  pure  form  of  limestone.  It  readily  gives 
up  pure  carbon  dioxide  when  it  is  treated  with  an  acid. 

Properties  of  Carbon  Dioxide.  Carbon  dioxide  is  colorless,  odor- 
less, and  has  a  slight  acid  taste,  is  somewhat  soluble  in  cold 
water,  and  becomes  very  soluble  under  high  pressure.  It  is 
about  one  and  one  half  times  as  heavy  as  air,  it  may  be  lique- 
fied by  pressure  at  ordinary  temperature,  and  it  may  be  frozen 
by  the  cold  produced  by  the  vaporization  of  the  liquid. 
Carbon  dioxide  neither  burns  nor  supports  combustion.  It  reacts 
feebly  with  water  to  form  a  weak  acid.  With  limewater  it 
reacts  to  form  a  white  precipitate  of  calcium  carbonate,  and 
this  reaction  is  a  distinctive  test  for  the  presence  of  carbon 
dioxide. 

An  important  use  of  carbon  dioxide  is  in  the  chemical  fire  ex- 
tinguisher. Dissolved  in  water  under  pressure,  it  is  an  essen- 
tial constituent  of  carbonated  beverages.  When  produced 
chemically  by  the  action  of  yeast  or  baking  powder  through- 
out a  mass  of  dough,  it  serves  in  raising  bread  and  cake. 

Carbon  monoxide  is  obtained  when  carbon  burns  with  insufficient 
oxygen.  It  contains  the  same  elements  as  carbon  dioxide, 
but  it  has  very  different  properties.  For  the  same  weight  of 
carbon,  carbon  dioxide  contains  just  twice  as  much  oxygen 
as  does  carbon  monoxide.  A  relation  of  this  kind  holds  for 
many  other  sets  of  compounds  besides  the  oxides  of  carbon, 
and  it  finds  general  expression  in  the  law  of  multiple  propor- 
tions. 

Questions 

1.  Enumerate  several  of  the  ways  in  which  carbon  dioxide  is 
being  continually  turned  into  the  atmosphere. 

2.  Explain  how  the  quantity  of  carbon  dioxide  in  the  atmosphere 
is  regulated  by  natural  processes  so  that  it  is  nearly  constant. 

3.  A  fragment  of  building  stone  when  dropped  into  a  test  tube 


66  THE  OXIDES  OF  CARBON 

containing  acid  effervesces  and  the  gas  given  off  clouds  limewater. 
What  probably  is  the  stone? 

4.  Upon  what  three  properties  of  carbon  dioxide  does  its  use  in 
fire  extinguishers  depend  ? 

5.  How  could  one  tell  whether  a  gas  that  would  not  support 
combustion  was  carbon  dioxide  or  nitrogen? 

6.  Why  is  it  dangerous  to  health  to  have  leaks  in  the  fire  pot 
of  a  hot-air  furnace? 

7.  If  the  water  of  a  mineral  spring  effervesces  strongly  on 
reaching  the  surface,  what  gas  would  you  suspect  and  how  would 
you  prove  the  point? 

8.  Why  is  it  customary  to  lower  a  lighted  candle  into  a  well 
before  sending  a  man  down  to  make  repairs? 

9.  Explain  two  different  methods  of  "  raising  "  bread. 

10.   What  is  one  of  the  principal  fuel  constituents  of  "  water 
gas"? 


CHAPTER  VII 


THE    ATMOSPHERE   AND    NITROGEN 

67.  Approximate  Composition  of  Air.     We  have  already 
seen   that   about   one  fifth   of  the  air  consists  of  oxygen, 
the  supporter  of  combustion,  and  that  after  that  one  fifth 
is  used  up  the  residual  gas  is  inert.     The  residual  gas  con- 
sists mainly  of  the  element  nitrogen. 

68.  Nitrogen  from  the  Air.     If  we  wish  to  obtain  fairly 
pure  nitrogen  from  air,  it  is  only  necessary  to  exhaust  the 
oxygen  by  causing  it  to  combine  with  some  substance  with 
which  it  gives  a  solid 

oxide,  for  example, 
with  iron,  copper,  or 
better,  phosphorus. 
Charcoal  would  not 
serve  the  purpose, 
because  its  combus- 
tion yields  a  gas 
which  would  remain 
admixed  with  the 
nitrogen. 

A  jarful  of  nitro- 
gen is  prepared  most 
readily  as 


„   ,,  FIG.    12. — Exhaustion  of   Oxygen.      Oxygen   re- 

lOllOWS  :   moved  from  air  under  a  bell   jar  by  means  of 


Place  a  piece  of  phos-   burning  phosphorus. 

phorus  on  a  porcelain  crucible  cover  which  rests  on  a  cork 
floating  on  the  surface  of  a  large   panful    of    water.     Set 

67 


68  THE  ATMOSPHERE   AND   NITROGEN 

fire  to  the  phosphorus  and  quickly  cover  it  with  a  large 
glass  bell  jar  (see  Fig.  12)  so  that  the  air  within  the  jar  is 
sealed  from  the  outer  air  by  means  of  the  water.  At  first 
the  heat  of  combustion  may  cause  so  much  expansion 
that  a  few  bubbles  of  air  will  be  forced  out  from  under 
the  sides  of  the  jar.  A  very  dense  white  smoke  is  formed 
but  this  should  not  be  mistaken  for  a  gas.  It  consists  simply 
of  very  fine  solid  particles  of  oxide  of  phosphorus  and  after 
a  sufficient  time  it  settles  out  completely  and  leaves  a 
perfectly  clear  gas.  As  the  gaseous  residue  cools,  the 
water  is  observed  to  rise  a  considerable  distance  in  the  bell 
jar,  indicating,  of  course,  that  there  is  now  less  gas  than 
formerly. 

If  enough  phosphorus  was  taken  at  the  outset  in  the  above 
experiment,  it  is  now  observed  that  some  of  it  remains  un- 
burned;  if  any  other  burning  material  is  now  introduced 
into  the  residual  gas,  it  is  extinguished,  showing  that  the  gas 
is  a  non-supporter  of  combustion.  When  we  first  found  that 
combustible  substances  burn  better  in  pure  oxygen  than  in 
air,  we  at  once  assumed  that,  mixed  with  the  oxygen  of  the 
air,  there  was  another  gas  which  was  a  non-supporter  of 
combustion.  We  have  now  shown  that  this  assumption  is 
correct. 

69.  Preparation  of    Pure  Nitrogen.      The   nitrogen  ob- 
tained by  removing  oxygen  from  air  has  small  quantities  of 
other  gases   mixed  with  it,  since  the  phosphorus   removes 
only  the   oxygen.     Pure   nitrogen   is   usually  prepared  by 
heating  a  concentrated  solution  of  ammonium  chloride  and 
sodium  nitrite.     The  gas  is  collected   in  bottles  by  water 
displacement. 

70.  Properties  of  Nitrogen.     On  testing  the  gas  in  the 
bottles,  it  is  found  that  it  neither  burns  nor  supports  com- 


AIR   A   MIXTURE  69 

bustion.  In  this  respect  it  resembles  carbon  dioxide,  but  it 
differs  from  that  gas  in  that  it  does  not  cloud  limewater. 
Nit^gen  is  a  little  lighter  than  air,  having  a  density,  at 
standard  conditions,  of  1.25  grams  per  liter.  Nitrogen  is 
only  half  as  soluble  in  water  as  oxygen  is.  100  volumes  of 
water  under  usual  conditions,  dissolve  about  1.6  volumes 
of  nitrogen. 

71.  Air  a  Mixture.  The  difference  in  solubility  of  the 
two  chief  components  of  the  air  furnishes  a  strong  reason  for 
believing  that  air  is  a  mixture  and  not  a  chemical  compound 
of  oxygen  and  nitrogen.  If  water  which  has  been  standing 
in  contact  with  the  air  is  boiled  and  the  dissolved  air  thereby 
expelled  is  caught  and  analyzed,  it  is  found  to  contain  34 
per  cent  of  oxygen,  whereas  atmospheric  air  contains  but  21 
per  cent.  If  air  were  a  chemical  compound,  it  would  be 
dissolved  as  a  whole  and  afterwards  on  removal  from  the 
solution  its  composition  would  not  be  changed. 

We  shall  see  in  a  later  chapter  that  there  are  several  dis- 
tinct compounds  of  oxygen  and  nitrogen,  the  properties  of 
which  are  radically  different  from  each  other  and  from  either 
of  the  uncombined  elements.  On  the  other  hand,  air  shows 
the  properties  of  both  oxygen  and  nitrogen,  only  modified 
to  the  extent  to  which  these  components  are  diluted  with 
one  another.  Thus  in  this  respect  also  air  shows  the  char- 
acteristics of  a  mixture. 

Furthermore,  oxygen  and  nitrogen  can  be  mixed  and 
the  mixture  at  once  shows  the  properties  of  air,  although  no 
evidence  of  chemical  reaction,  such  as  evolution  of  heat, 
is  to  be  observed.  The  mixture  indeed  can  be  made  in  pro- 
portions other  than  those  existing  in  air  and  yet  the  proper- 
ties are  not  essentially  different.  If  air  were  a  chemical 
compound,  its  composition  would  be  unalterable. 


70  THE   ATMOSPHERE   AND   NITROGEN 

72.  Inertness  of  Free  Nitrogen  and  Importance  of  Com- 
bined Nitrogen.     The  above  described  simple  experiments 
with  the  bottle  of  nitrogen  have  shown  that  nitrogen  is  a 
very  inert  gas.     Nitrogen  can,  however,  enter  into  chemical 
combination,  and    many    important    compounds,   including 
ammonia  and  nitric  acid,  are  formed  from  it.     Nitrogen  is  an 
essential  constituent  of  all  living  organisms;  indeed  human 
food    is    judged    largely    according    to    the    percentage    of 
chemically    combined    nitrogen    which    it    contains ;    and 
soils  and  fertilizers  are  of  value  as  plant  foods  largely  in 
proportion   to   the  amount  of  chemically  combined  nitro- 
gen which  they  contain.     In  spite  of  the  great  importance 
of  nitrogen  in  the  chemistry  of  living  things,  the  atmos- 
pheric nitrogen  plays  almost  no  direct  part  in  the  nutri- 
tion of  animals  and  plants.     This  fact  further  emphasizes 
the  inert  nature  of  this  element  when  it  is  in  the  uncom- 
bined  state.     Of  the  air  that  men  and  animals  breathe,  it 
is  only  the  oxygen  that  is  utilized ;  the  nitrogen  is  entirely 
exhaled  again. 

73.  Nitrogenous  Foods.     All  the  nitrogen  for  our  nutri- 
tion enters  our  bodies  as  combined  nitrogen  in  our  food. 
Meat  and  fish  foods,  milk  and  cheese,  beans  and  peas  and 
mushrooms,  contain  a  considerable  proportion  of  combined 
nitrogen ;    most  grains   and  vegetables  contain   less.     The 
gluten  of  wheat,  which  comprises  some  twelve  to  fifteen  per 
cent  of  the  weight  of  the  whole  wheat,  is  a  nitrogen-containing 
food,  and  it  is  largely  due  to  the  quantity  of  gluten  that 
wheat  is  so  highly  prized  as  a  food. 

Just  as  animals  are  compelled  to  take  the  nitrogen  that 
they  need  in  the  combined  form,  so  plants  obtain  theirs  from 
the  soil  in  the  form  of  soluble  nitrogen  compounds  which  are 
taken  up  with  the  water  of  the  soil  through  their  roots.  It 


FIXATION   OF  NITROGEN  71 

is  on  account  of  the  nitrogen  which  it  contains  that  slaughter- 
house refuse  is  valuable  for  fertilizer. 

74.  Fixation  of  Nitrogen  by  means  of  Legumes.     There 
are  a  few  plants,  namely,  those  of  the  legume  family,  of  which 
beans,  peas,  and  clover  are  members,  which  are  able  through 
the  agency  of  colonies  of  bacteria  which  live  on  their  roots, 
to  obtain  a  certain  amount  of  atmospheric  nitrogen  by  caus- 
ing it  to  enter  chemical  combination.     It  is  on  this  account 
that  when  a  field  is  exhausted  of  its  nitrogenous  material 
by  long  cultivation,  the  farmer  plants  it  with  clover  to  re- 
store its  fertility. 

75.  Artificial  Methods  of  Causing  Nitrogen  to  Combine. 
We  have  seen  that  the  natural  means  of  "  fixing  nitrogen," 
as  it  is  called,  that  is,  of  causing  the  inert  gas  of  the  air 
to  enter  chemical  combination,  in  which  state  it  is  avail- 
able for  nutrition,  are  very  limited.     Within  recent  years 
artificial  means  have  been  developed,  and  now  the  fixation 
of  nitrogen  directly  from  the  air,  for  use  as  a  fertilizer,  is 
becoming  an  industry  of  importance.     Indeed,  this  industry 
promises  to  increase  to  a  very  great  extent. 

One  of  the  most  important  of  the  artificial  methods  de- 
pends on  a  fact  discovered  by  Cavendish  as  long  ago  as  the 
latter  part  of  the  eighteenth  century.  He  passed  electric 
sparks  through  a  confined  volume  of  air  and  found  that  the 
"  air  "  would  then  dissolve  in  small  amount  in  water  to  form  an 
acid  (nitric  acid).  Under  the  influence  of  the  electric  spark 
nitrogen  arid  oxygen  combine  to  form  nitric  oxide.  This 
oxide  of  nitrogen  is  a  chemical  compound;  it  is  no  longer  a 
mixture  of  oxygen  and  nitrogen,  but  an  entirely  new  sub- 
stance with  entirely  new  properties.  The  important  new 
property  from  the  standpoint  of  this  process  is  that  it  unites 
readily  with  water  and  more  oxygen  to  form  nitric  acid. 


72  THE  ATMOSPHERE   AND   NITROGEN 

For  use  as  a  fertilizer  the  acid  is  treated  with  limestone 
whereby  calcium  nitrate  is  obtained.    . 

A  large  amount  of  electrical  energy  is  expended  in  effecting 
this  chemical  combination  of  nitrogen,  and  it  is  therefore  only 
near  great  waterfalls,  where  power  is  abundant,  that  this 
process  has  a  chance  of  success.  Niagara  Falls  being  in  a 
thickly  settled  district  where  the  demand  for  power  is  great, 
the  price  is  not  as  low  as  one  might  expect  from  the  magni- 
tude of  the  supply.  Norway  possesses  great  waterfalls  where 
electric  power  is  developed  so  cheaply  that  the  manufacture 
of  nitric  acid  and  nitrates  from  the  air  is  proving  a  distinct 
commercial  success. 

76.  Carbon  Dioxide  and  Water  Vapor  in  Air.     Oxygen 
and  nitrogen  constitute  the  main  bulk  of  the  atmosphere, 
and  they  are  perhaps  the  most  important  constituents  of 
the  air ;    but  there  are  two  other  constituents  which  are  of 
exceedingly  great  importance,  namely,  carbon  dioxide,  which 
is  only  present  to  the  extent  of  three  or  four  volumes  in  10,000 
volumes  of  air,  and  water  vapor,  which  is  present  in  variable 
amount    according   to    atmospheric    conditions,  but    rarely 
exceeds  2  per  cent  of  the  whole. 

77.  Argon.     In   addition   to   these   gases,    there   is    still 
another,  whose  presence  was  not  even  suspected  until  1894. 
It  is  of  no  practical  importance,  but  it  is  of  very  great  interest 
on  account  of  the  fact  that  it  constitutes  nearly  1  per  cent  of 
the  air  and  yet  it  remained  so  long  undiscovered. 

If  the  residual  part  of  the  air  from  which  oxygen  has  been 
extracted  is  subjected  to  the  action  of  magnesium  and  the 
electric  spark,  the  nitrogen  combines  to  form  magnesium 
nitride.  When  this  is  done  there  is  always  left  a  small 
amount  of  gas,  amounting  to  about  1  per  cent  of  the  air 
taken  at  first.  This  fact  was  observed  by  Cavendish,  who 


ARGON  73 

noticed  that  whenever  he  attempted  to  remove  all  the  oxygen 
and  nitrogen  from  a  confined  volume  of  air,  a  small  bubble 
of  gas  always  remained.  Cavendish  observed  this  fact,  but 
paid  little  attention  to  it,  probably  thinking  that  the  gas 
remaining  was  some  nitrogen  which  had  not  been  completely 
removed.  Indeed,  this  residual  gas  shows  the  same  general 
property  of  inertness  as  nitrogen.  It  thus  escaped  serious 
attention  until  1894.  In  that  year  Lord  Rayleigh,  an  English 
physicist,  made  accurate  measurements  of  the  density  of 
nitrogen  and  other  gases.  He  observed  that  the  result 
with  nitrogen  obtained  from  air  was  different  from  that  with 
nitrogen  obtained  by  decomposing  a  chemical  compound  of 
nitrogen.  The  fact  was  then  recalled  that  a  residue  was 
always  left  when  the  nitrogen  residue  of  air  was  sparked 
with  magnesium,  and  Rayleigh  concluded  that  there  must 
be  another,  previously  unsuspected,  gas  in  the  air.  Ray- 
leigh was  not  a  chemist  and  did  not  feel  competent  to 
carry  the  study  of  the  problem  further;  he  therefore  en- 
listed the  aid  of  the  noted  chemist,  Sir  William  Ramsay, 
who  after  extensive  study  confirmed  the  discovery  of 
another  constituent  of  the  air,  and  declared  further  that  this 
gas  was  a  new  element.  It  was  named  argon,  meaning  inert. 

Argon  constitutes  about  1  per  cent  by  volume  of  the  air. 
Besides  argon,  Ramsay  discovered  four  other  new  gases, 
helium,  neon,  krypton,  and  xenon  which,  however,  are  present 
in  the  air  in  exceedingly  minute  quantities. 

The  most  remarkable  property  of  argon,  as  well  as  of  the 
four  other  more  recently  discovered  gases,  is  that  of  absolute 
chemical  inertness.  These  gases  can  be  made  to  combine 
with  no  other  element,  and  this  accounts  for  their  being  so 
long  undiscovered  and  mistaken  for  a  slight  residue  of  in- 
active nitrogen.  Physically,  argon  is  not  very  different 


74  THE  ATMOSPHERE  AND   NITROGEN 

from  nitrogen.  It  is  colorless,  odorless,  and  tasteless;  it  is 
heavier  than  nitrogen,  which  fact,  as  we  have  seen,  led  to  its 
discovery ;  its  boiling  point  is  higher  than  that  of  nitrogen 
and  lower  than  that  of  oxygen. 

ACCURATE  DETERMINATION  OF  THE  VOLUME  PER  CENT  OF 
OXYGEN  IN  AIR 

78.  In  order  that  an  accurate  knowledge  of  the  composi- 
tion of  the  air  may  be  obtained,  a  much  more  careful  method 
of  removing  the  oxygen  must  be  employed  than  that  de- 
scribed in  the  first  part  of  this  chapter.  There,  it  will  be  re- 
membered, it  was  difficult  to  prevent  the  escape  of  some  of 
the  confined  gas  in  consequence  of  the  expansive  effect  of  the 
heat  of  the  burning  phosphorus. 

To  carry  out  an  analysis  of  the  air,  it  is  necessary  to  start 
with  an  accurately  measured  quantity,  to  remove  all  of  the 
oxygen  wrhile  care  is  taken  to  lose  none  of  the  nitrogen,  and, 
finally,  to  measure  the  volume  of  the  residual  gas.  The  final 
volume  is,  of  course,  the  sum  of  the  volumes  of  the  nitrogen 
and  argon.  The  quantity  of  carbon  dioxide  —  only  yf  -$  of  1 
per  cent  of  the  whole  —  is  so  small  as  to  be  negligible.  The 
water  vapor  in  the  air  may  also  be  neglected  if  the  volumes 
measured  are  all  uniformly  confined  over  water. 

Let  us  now  follow  out  step  by  step  the  course  of  an  actual 
analysis  of  air  such  as  we  might  perform  in  the  chemical 
laboratory. 

A  narrow  cylindrical  glass  tube  which  holds  100  c.c.  and 
which  is  graduated  to  tenths  of  a  cubic  centimeter  is  used  to 
contain  and  to  measure  the  air.  The  cylinder  is  inverted 
and  its  open  end  is  sunk  beneath  the  surface  of  a  deep 
body  of  water  in  a  tall  glass  jar.  A  definite  volume  of 
air  is  thus  confined  in  the  measuring  tube.  If  a  few 


VOLUME   PER  CENT  OF  OXYGEN   IN   AIR 


75 


cubic  centimeters  of  water  are  poured  into  the  cylinder 
before  inverting  it,  the  air  volume  will  not  reach  to  the 
lower  end  of  the  tube,  but  its  boundary  will  come  where 
it  can  be  seen  behind  the  graduations.  The  tube  is  now 
raised  or  lowered  until  the  water  levels  inside  and  outside 
are  the  same.  There  is  thus  no 
pressure  on  the  gas  due  to  dif- 
ference in  water  level.  If  there 
were  any  such  difference,  it 
would  have  to  be  taken  into  ac- 
count; therefore  it  is  simpler  to 
bring  the  levels  both  to  the 
same  point  and  thus  avoid  the 
complication.  The  volume  is 
noted  as  95.9  c.c.  At  the  same 
time  the  atmospheric  pressure 
is  noted  as  778  mm.  of  mer- 
cury, and  the  temperature  as 
21°  C.  The  necessity  for  the 
latter  readings  will  appear  a 
little  later.  Now  that  the  defi- 
nite amount  of  air  is  measured, 
we  proceed  to  remove  the 
oxygen  by  inserting  from  under- 
neath  a  piece  of  yellow  phos-  F'°- 
phorus  attached  to  a  bent  piece 
of  slender  wire  (Fig.  13).  This  wet  phosphorus  does  not  take 
fire  in  the  tube,  but  it  does  combine  slowly  with  oxygen  and 
if  time  enough  is  allowed,  it  removes  the  oxygen  as  com- 
pletely as  if  it  had  flamed  up  in  rapid  combustion.  It  is 
therefore  necessary  to  leave  the  whole  apparatus  over  night. 
The  next  morning  it  is  extremely  probable  that  the  oxygen  has 


76 


THE  ATMOSPHERE  AND   NITROGEN 


been  completely  exhausted.  The  remaining  lump  of  phos- 
phorus is  removed  and  the  readings  taken  as  before.  They 
are  :  volume,  77.2  c.c. ;  pressure,  756 
mm. ;  and  temperature,  18°  C.  To 
prove,  now,  that  no  oxygen  is  left  in 
the  tube,  a  fresh  piece  of  phosphorus, 
which  will  act  more  vigorously  than 
the  old  piece,  should  be  inserted  and 
allowed  to  stand  an  hour  or  two. 
This  is  then  removed  and  the  volume 
is  again  read  and  found  to  be  77.2  c.c. 
This  reading,  being  the  same  as 
the  second,  shows  that  no  oxygen  has 
been  absorbed  by  the  second  piece 
of  phosphorus.  The  thermometer  and 
barometer  are  again  read  and  found 
to  be  18  degrees  and  756  mm.,  re- 
spectively, not  having  changed  since 
the  previous  reading.  (If  they  had 
changed  a  little,  it  might  have  caused 
some  slight  change  in  the  volume  of  the 
gas  not  due  to  further  absorption  of 
oxygen.) 

Since  we  are  compelled  to  leave  the 
experiment  over  night,  a  complication 
arises  which  would  not  bother  us  if 
all  the  readings  could  be  taken  at 
about  the  same  time.  The  volume  of 
any  gas  changes  with  changes  of 
temperature  and  pressure.  Now  the 
temperature  of  the  gas  we  are  interested  in  will  probably  not 
be  the  same  the  next  day  as  at  the  time  when  the  experi- 


FIG.  14.  —  Barometer. 
The  pressure  of  the  at- 
mosphere on  the  sur- 
face of  the  mercury  in 
the  cup  counterbalances 
the  weight  of  the  column 
of  mercury  in  the  tube. 
The  height  of  the  top  of 
the  mercury  surface  in 
the  tube  above  the  free 
surface  in  the  cup  thus 
measures  the  pressure  of 
the  atmosphere.  The 
scale  and  measuring  de- 
vice is  not  shown  in  the 
figure. 


SUMMARY  77 

merit  was  started;  neither  will  the  pressure  of  the  atmos- 
phere upon  the  water  in  the  deep  jar  (and  hence  on  our  air 
which  is  confined  above  the  water)  be  the  same  the  follow- 
ing day.  Hence  the  necessity  of  noting  the  temperature 
and  pressure  at  the  time  of  each  reading  of  volume.  The 
temperature  may  be  found  by  hanging  a  thermometer  in 
the  water,  if  the  latter  has  been  drawn  long  enough  to  come 
to  the  temperature  of  the  room.  The  pressure  of  the  air  is 
found  by  reading  the  barometer  (see  Fig.  14). 

In  making  our  calculations,  of  the  composition  of  the  air, 
it  will  not  do  to  subtract  the  77.2  c.c.  from  the  95.9  c.c.  and 
claim  that  18.7  c.c.  of  oxygen  were  removed,  for  the  tempera- 
ture and  the  pressure  both  changed  between  the  readings. 
We  must  first  learn  what  effect  change  in  temperature  and  in 
pressure  have  upon  the  volume  of  a  gas  and  then  correct  the 
observed  volumes  for  these  changes.  The  next  chapter  is 
devoted  to  the  effect  of  conditions  on  the  volume  of  gases. 

SUMMARY 

Components  of  Air.  The  air  is  a  physical  mixture  of  several  gases, 
of  which  about  one  fifth  by  volume  is  oxygen  and  nearly  all 
of  the  other  four  fifths  is  nitrogen. 

About  one  per  cent  of  the  air  consists  of  argon,  an  absolutely  inert 
gas.     Besides  argon,  there  are  four  other  similarly  inert  gases 
in  very  minute  amounts. 
Air  contains  a  small  amount  of  carbon  dioxide,  but  this  small 

component  is  of  very  great  importance.  * 

Water  vapor  is  present  in  the  air  in  varying  proportions,  but 
rarely  exceeding  two  per  cent.  Its  presence  is  of  great  im- 
portance. 

Nitrogen  an  Important  Plant  Food.  Nitrogen  is  very  inert  chemi- 
cally and  can  be  made  to  combine  with  other  elements  only 
with  difficulty.  All  plant  and  animal  life  requires  nitrogen 
for  its  nutrition,  but,  with  the  exception  of  plants  of  the 


78  THE   ATMOSPHERE   AND   NITROGEN 

legume  family,  which  get  nitrogen  from  the  air  by  the  aid  of 
colonies  of  bacteria  growing  on  their  roots,  all  plant  and  animal 
life  is  compelled  to  use  nitrogen  that  is  already  in  the  com- 
bined state.  Combined  nitrogen  is,  therefore,  an  essential 
constituent  of  fertilizers.  Meats  and  fish,  cheese  and  milk, 
beans  and  peas,  and  to  some  extent  grains  and  vegetables, 
supply  us  with  the  necessary  nitrogen  in  our  foods.  Artifi- 
cial methods  of  causing  the  atmospheric  nitrogen  to  combine 
are  assuming  commercial  importance  in  connection  with  the 
manufacture  of  nitrogenous  fertilizers. 

Questions 

1.  Why  is  air  considered   a   mixture   rather  than   a  definite 
chemical  compound  ?     Give  facts  which  show  this,  other  than  those 
mentioned  in  Section  71. 

2.  Name  in  the  order  of  their  abundance  the  gases  present  in 
the  air. 

3.  Discuss  the  relative  importance  of  these  gases. 

4.  Explain  why  the  fixation  of  nitrogen  is  difficult,  but  of  great 
importance. 

5.  Describe  processes  by  which  atmospheric  nitrogen  is  in  nature 
brought  into  a  state  of  combination. 

6.  Describe  artificial  methods  of  accomplishing  the  same  thing. 

7.  Give  three  methods  by  which  oxygen  can  be  removed  from 
the  air. 

8.  Why  would  it   be   advantageous  from   the   standpoint   of 
economy  of  heat  to  use  pure  oxygen  rather  than  air  in  burning  coal 
under  boilers?     Why  is  pure  oxygen  not  thus  used  at  present? 

9.  When  meat   is   excessively   high   priced,   what   substitutes 
which  would  furnish  nearly  or  quite  as   much  available  nitrogen 
might  be  used  as  food  ? 

10.   Why  do  farmers  plant  clover  or  alfalfa  on  fields  which  have 
become  exhausted  from  frequent  raising  of  wheat? 


CHAPTER  VIII 

THE    GAS  LAWS 

79.  It  is  a  familiar  fact  that  air  and  other  gases  expand 
when  heated  and  contract  when  cooled.     The  expansion  of 
the  air  over  the  torrid  zone  and  the  resulting  trade  winds  is 
an  example  familiar  from  the  study  of  geog- 
raphy.    A  similar    phenomenon    on   a    small 

scale  is  observed  in  the  rise  of  hot  air  over  a 
radiator. 

The  expansion  and  contraction  of  air  may 
be  readily  demonstrated  if  a  flask  is  closed 
with  a  one-holed  rubber  stopper  and  through 
the  latter  is  passed  a  narrow  glass  tube  in  the 
middle  of  which  a  short  column  of  colored 
liquid  is  caught  so  as  to  indicate  the  boundary 
of  the  definite  volume  of  confined  air.  The  air 
expands  even  with  the  slight  warmth  imparted 
by  holding  the  hands  against  the  sides  of  the 
flask,  and  the  thread  of  liquid  is  seen  to  move 
up  in  the  tube.  On  the  other  hand,  the  vol-  FIG.  15. —Gas 
ume  of  air  contracts  if  the  flask  is  cooled,  and  Volumes  change 

with  Temperature. 

the   thread  of  liquid   can   be   seen   to    move 

downward.     Exactly  the  same  behavior  is  observed  if  the 

flask  is  filled  with  any  other  gas  in  place  of  air. 

80.  Measurement   of   Temperature;    Centigrade    Scale. 
In  all  scientific  work,  temperature  is  measured  by  a  scale 
known  as  the    centigrade  scale.     This  scale  possesses  two 

B.  AND  W.  C'HEM. 6  79 


80  THE  GAS  LAWS 

fixed  points  which  are  very  easy  to  duplicate;  namely,  the 
temperature  of  melting  ice  and  the  temperature  of  the  steam 
from  water  boiling  under  a  pressure  equal  to  the  average 
atmospheric  pressure  at  sea  level.  The  first  point  is  called 
zero  degrees  and  the  second  point  is  called  100  degrees; 
and  the  interval  between  these  two  points  is  divided  into 
100  parts  (hence  the  name  centigrade,  from  the  Latin,  indi- 
cating that  the  scale  is  graduated  into  one  hundred  parts). 
By  extending  spacings  of  equal  lengths  below  0  degrees  and 
above  100  degrees,  the  centigrade  thermometer  is  made  to 
read  over  all  ranges  of  temperature. 

81.  When  it  is  wished  to  express  the  quantity  of  a  gas  in 
terms  of  its  volume,  it  is  clearly  necessary  to  state  the  tem- 
perature of  the  gas  at  the  time  the  volume  was  measured. 
One  liter  of  a  gas  at  100°  C.  would  have  a  different  weight 
from  one  liter  of  the  same  gas  at  0°  C. 

If  gases  were  not  so  difficult  to  weigh,  we  might  perhaps 
always  express  the  quantity  of  a  gas  by  weight;  but  on 
account  of  the  extreme  lightness  of  gases,  it  is  difficult  to 
weigh  them  accurately.  It  is,  however,  very  easy  to  meas- 
ure the  volume  of  a  gas. 

It  has  been  found  that  for  all  gases  the  change  in  volume 
for  change  in  temperature  is  regular,  namely,  for  an  in- 
crease of  1°  C.  the  increase  in  volume  is  equal  to  %j%  of  the 
volume  at  0°  C.,  or  for  a  decrease  of  1°  of  temperature, 
the  decrease  is  likewise  2  TIT  °f  the  volume  at  0°  C.  This 
uniformity  in  the  rate  of  change  was  first  noted  by  Charles, 
in  1787,  and  his  name  is  now  given  to  the  law  which  connects 
the  change  of  volume  with  the  change  of  temperature. 

82.  Absolute  Zero.     Before  we  take  up  the  study  of  the 
application  of  Charles'  law,  it  will  be  necessary  to  consider 
briefly  the  matter  of  absolute  temperatures,  since  the  modern 


THE   ABSOLUTE  SCALE  OF  TEMPERATURE 


81 


treatment  of  Charles'  law  makes  use  of 
the  absolute  scale  of  temperature.  Let 
Figure  16  represent  a  vessel  filled  with 
a  gas  at  0°  C.  Suppose  the  piston  with 
which  the  top  is  closed  to  be  gas-tight 
and  to  slip  up  and  down  without  fric- 
tion. If,  now,  the  gas  is  cooled  by 
1°  C.,  the  volume,  according  to  the  rule 
just  stated,  will  decrease  ^y^,  and  the 
piston  will  sink  to  a  position  shown  by 
the  first  dotted  line.  If  it  is  again 
cooled  by  1°  C.,  that  is  to  -  2°,  the 
volume  will  again  be  diminished  by  an 
amount  equal  to  ^yj  of  its  volume  at 
0°  C.,  and  the  piston  will  sink  to  the 
second  dotted  line.  It  can  readily  be 
seen  that  when  the  temperature  has 
been  decreased  to  273  degrees  below 
zero,  the  volume  will  have  decreased 
to  zero  if  the  same  regularity  in  -be- 
havior has  continued.  This  value, 
—  273  degrees  centigrade,  has  a  great 
theoretical  importance  and  it  is  what  is 
known  as  the  absolute  zero  of  tem- 
perature. 

83.  The  absolute  scale  of  tempera- 
ture is  graduated  in  degrees  which  are 
equal  to  those  on  the  centigrade  ther- 
mometer, but  the  zero  point  is  273 
degrees  lower  than  that  of  the  centi- 
grade scale.  This  being  the  case,  it  is 
necessary,  whenever  we  wish  to  convert 


11 


o*c 

t-  -5'C 
HO'C 

«-  -20°C 


-273'C 


FIG.  16.  —  Showing 
Change  of  Volume  of  Gas 
with  Changing  Tempera- 
ture. The  volumes 
marked  off  on  the  lower 
part  of  the  cylinder 
show  the  behavior  of 
the  idealy  perfect  gas. 
Of  course  no  actual  gas 
conforms  perfectly  to 
this  ideal  at  very  low 
temperatures  although 
helium  comes  fairly 
near  to  it. 


82 


THE   GAS  LAWS 


-too* 


-373-  - 


a  centigrade  reading  into  absolute,  to  add  273  degrees  to 
the  centigrade  reading.     The  adding  must  be  algebraic  in 

order  to  take  account 
properly  of  any  minus  read- 
ings. For  example,  to 
change  20°  centigrade  to 
absolute,  20  +  273  =  293. 
Hence  20°  C.  equals  293° 
absolute.  Again,  to  change 
-  20°  C.  to  absolute :  -  20 
+  273  =  253°  absolute. 
That  is,  if  the  absolute 
zero  is  273  degrees  below 
the  centigrade  zero,  and 
if  the  temperature  —  20°  C. 
is  20  degrees  below  the 
centigrade  zero,  then 
-  20°  C.  is  253  degrees 
above  the  absolute  zero. 

84.   Charles'  Law.     Now 
if  any  volume  of  gas  —  say, 
for  the  sake  of  convenience, 
273    c.c.  —  were   taken   at 
0°  C.,  which    equals    273° 
absolute,    and    cooled,   say 
10°,  to  263°  absolute,  then,  according  to  the  rule  previously 
mentioned,  it  would  shrink  by  ffls  °f  its  volume  at  zero,  or 
in  this  case  by  ^VV  of  the  273  c.c.  which  we  chose  for  con- 
venience to  take.     Hence  the  new  volume  would  be 
273  c.c.  -  (^  of  273  c.c.) 
=  273  c.c.  -  10  c.c. 
=  263  c.c. 


-275°.  _JLO'__ 
FAHRENHEIT     CENTIGRADE    ABSOLUTE 

FIG.   17.  —  Comparison  of  Different  Ther- 
mometric  Scales. 


CORRECTION  OF  GAS  VOLUMES          83 

It  will  be  noticed  that  the  new  volume  in  this  case  numeri- 
cally equals  the  new  temperature  on  the  absolute  scale.  If 
some  volume  other  than  273  c.c.  had  been  taken,  the  new 
volume  would  not  have  equaled  the  new  temperature  numeri- 
cally, but  a  simple  proportion  would  have  existed  between 
the  two  temperatures  and  the  two  volumes ;  hence  a  simple 
statement  of  the  law  of  the  effect  of  temperature  on  gas 
volumes,  in  terms  of  the  absolute  temperature,  is  as  follows  : 

//  the  pressure  upon  a  gas  remains  constant,  the  volume  varies 
in  direct  proportion  to  the  absolute  temperature. 

The  law  in  this  form  will  be  found  simpler  to  use  in  calculat- 
ing corrections  of  gas  volumes  than  in  the  form  which  states 
that  the  change  in  volume  per  one  degree  change  in  tempera- 
ture is  2T3-  °f  the  volume  at  0°  centigrade. 

85.  Example  of  a  Calculation  based  on  Charles'  Law. 
In  the  experiment  to  determine  the  per  cent  of  oxygen  in  air 
(Chapter  VII,  p.  74),  there  were  95.9  c.c.  of  air  in  the  gas 
measuring  tube  at  the  start.  The  next  day,  after  the  phos- 
phorus had  taken  out  the  oxygen,  there  were  77.2  c.c.  of 
residual  gas.  The  temperature  at  the  time  the  first  reading 
was  taken  was  21°  C.  The  next  day,  when  the  volume  was 
read,  the  temperature  was  18°  C.  To  make  a  fair  comparison 
between  the  two  volumes  they  must  either  be  brought  to  the 
same  temperature,  or,  what  is  more  easily  done,  be  calculated 
to  the  same  temperature  by  the  use  of^Charles'  law.  Let 
us  change  the  first  volume  to  the  volume  it  would  occupy  at 
the  temperature  of  the  final  reading.  We  will  first  change 
the  centigrade  readings  to  absolute :  21  +  273  =  294 ; 
18  +  273  =  291.  "  The  problem  then  becomes  :  the  volume 
of  the  gas  is  95.9  c.c.  at  294°  absolute.  What  will  be  the 
volume  at  291°  absolute?  "  According  to  Charles'  law  the 
volume  varies  directly  as  the  absolute  temperature,  that  is, 


84  THE  GA$  LAWS 

the  two  volumes  and  the  two  temperatures  are  in  simple 
proportion. 

Then  95.9  =  294 

x          291 

where  x  stands  for  the  new  volume.     Multiplying  the  means 
and  the  extremes  of  the  proportion,  we  have  : 

294  x  =  95.9  X  291  =  27907 


whence  x  =  -  94.9 


which  is  the  number  of  cubic  centimeters  in  the  new  volume. 

We  now  might  subtract  the  77.2  c.c.  of  residual  gas  from 
the  94.9  c.c.  to  find  how  much  oxygen  had  been  removed, 
provided  only  that  the  pressure  on  the  gas  had  remained 
constant  over  night,  but  the  pressure  did  not  remain  constant. 
We  shall,  therefore,  need  to  consider  how  changes  of  pressure 
affect  the  volume  of  a  gas. 

86.  Effect  of  Pressure  on  Volume  ;  Boyle's  Law.  It  is  a 
fact  well  known  to  all  who  have  used  a  bicycle  pump  that  air 
decreases  in  volume  under  increased  pressure,  and  that  it 
increases  in  volume  if  the  pressure  is  diminished.  It  has 
been  found  by  accurate  measurements  that  if  the  temperature 
is  kept  constant,  the  volume  of  a  definite  quantity  of  gas  varies  in 
inverse  proportion  to  the  pressure.  This  rule  holds  for  all 
gases,  and  it  is  known  as  Boyle's  law.1 

Illustration  of  Boyle's  law  :  If  the  pressure  upon  one  liter 
of  gas  in  a  cylinder  with  an  air-tight  piston  is  doubled,  the 
volume  decreases  to  one  half  liter.  If,  on  the  other  hand, 

1  For  accurate  work  at  extremely  high  pressures,  this  law  does 
not  hold  strictly  true  ;  neither  does  the  law  of  Charles  hold  strictly 
true  under  all  conditions;  nevertheless,  for  all  ordinary  purposes 
these  two  laws  may  be  regarded  as  correct. 


CORRECTION  OF  GAS  VOLUMES  85 

the  pressure  is  lowered  to  one  half  its  initial  amount,  the 
volume  increases  to  two  liters. 

87.  To  return,  now,  to  the  calculations  of  our  experiment 
on  the  volume  composition  of  air,  we  found  on  page  84  that 
the  95.9  c.c.  of  air  with  which  we  started  would  have  had  a 
volume  of  94.9  c.c.  at  the  temperature  of  the  final  measure- 
ment ;  but  we  did  not  take  into  account  the  change  of  pres- 
sure. That  we  must  do.  The  pressure  as  read  on  the 
barometer  on  the  first  day  was  778  mm.  and  on  the  second  day 
it  was  756  mm.  It  is  therefore  necessary  to  correct  the  94.9 
c.c.  for  the  change  of  pressure  from  778  mm.  to  756  mm.  in 
order  to  find  what  volume  the  air  originally  taken  would 
have  had,  if  it  had  been  measured  under  the  same  conditions 
as  the  residual  gas.  The  calculation  is  as  follows : 

94.9  =  756 
x         778 

whence,  multiplying  means  and  extremes: 
756  x  =  94.9  X  778  =  73832 
and  x  =  73832/756  =  97.7 

Therefore  the  new  volume  is  97.7  c.c. 

We  have  now  calculated  our  initial  air  volume  to  the  same 
conditions)1  as  those  of  the  residue,  and  we  can  subtract  the 
volume  of  residue  from  that  of  air  to  find  the  volume  of 
oxygen  removed  by  the  phosphorus. 

97.7  -  77.2  =  20.5 

1  In  all  of  these  measurements  the  gases  are  measured  over  water 
and  hence  are  saturated  with  water  vapor.  The  amount  of  water 
vapor  present  is  nearly  the  same  in  every  case  if  the  measurements 
are  all  made  at  about  room  temperature,  and  hence  the  errors  due 
to  neglecting  it  very  nearly  compensate  each  other.  (See  footnote 
on  page  86.) 


86  THE  GAS  LAWS 

Thus  20.5  c.c.  of  oxygen  have  been  removed.  Next  to  find 
the  volume  per  cent  of  oxygen  divide  20.5  by  97.7.  The 
quotient  is  0.210  and  thus  21.0  per  cent  by  volume  of  the  air 
is  shown  to  be  oxygen.  This  result  is  of  approximate  accu- 
racy and  a  result  obtained  by  this  method  should  not  vary 
by  more  than  0.2  of  one  per  cent  from  the  true  value. 

88.  Standard     Conditions.     We    most     frequently    use 
Charles'  and  Boyle's  laws  for  the  purpose  of  changing  from 
ordinary  conditions  of  temperature  and  pressure  to  0°  C.  and 
760  mm.     These  are  called  standard  temperature  and  standard . 
pressure  and  together  are  often  spoken  of  as  standard  condi- 
tions.1 

The  standard  temperature  is  chosen  and  agreed  upon  by  all 
scientific  workers  as  0°  C.  because  this  is  the  temperature  of 
melting  ice  and  is  the  most  easily  and  accurately  obtainable 
temperature. 

The  standard  pressure  is  taken  as  760  mm.  of  mercury 
because  that  is  found  to  be  the  average  pressure  of  the 
atmosphere  at  sea  level. 

89.  Calculations   to    Standard    Conditions.     The  weight 
per  liter  of  all  gases  at  standard  conditions  has  been  very 
carefully  determined;  and  in  most  cases,  when  we  wish  to 
know  the  weight  of  any  gas  that  we  may  collect,  we  measure 
its  volume,  take  its  temperature  and  pressure  and  calculate 

1  The  weight  of  a  gas  at  standard  conditions  always  refers  to  dry 
gas.  If  the  gas  is  collected  over  water  at  room  temperature  and 
atmospheric  pressure,  it  will  occupy  between  two  and  three  per  cent 
more  volume  than  if  it  were  collected  dry.  In  connection  with  the 
study  of  this  chapter,  the  student  is  recommended  to  ignore  the 
possible  presence  of  water  vapor. 

For  those  who  have  time  to  study  the  matter  or  the  necessity 
for  so  doing,  the  method  for  correcting  for  the  presence  of  water 
vapor  is  outlined  on  page  426,  in  the  Appendix. 


CALCULATIONS  TO  STANDARD   CONDITIONS  87 

what  volume  it  would  have  at  standard  conditions  and  then 
calculate  its  weight  by  comparing  its  volume  with  the  unit 
volume  (the  liter)  whose  weight  is  known.  The  method  of 
calculating  to  standard  conditions  is  precisely  similar  to  that 
for  calculating  to  any  other  conditions.  The  volume  varies 
directly  with  the  absolute  temperature  and  inversely  with  the 
pressure.1 

For  example  —  let  the  observed  volume  be  45.0  c.c.,  the 
temperature,  as  read,  be  20°  C.,  and  the  pressure  be  740  mm. 
The  gas  may  be  supposed  to  be  dry  in  this  case.  If  it  were 
wet,  a  further  correction  would  be  necessary  in  accurate  work, 
as  the  water  vapor  has  a  pressure  of  its  own  which  has  to  be 
taken  into  account.  We  wish  to  find  the  volume  at  0°  C.  and 
760  mm.  The  steps  in  the  calculation  are  (correcting  first 
for  the  temperature  change) : 

45.0  =  273  +  20  =  293 
x        273  +  0        273 

then,  multiplying  means  and  extremes,  we  have 

293  x  =  45  X  273 
whence,  dividing  through  by  293,  we  have 

_  45  X  273 
293 

We  might  perform  the  indicated  operations  and  thus 
find  the  numerical  value  of  x,  which  would  be  the  new 
volume  after  the  temperature  change ;  but  we  do  not  care  to 
know  that,  but  rather  the  volume  after  both  temperature  and 

45  X  273 
pressure   have    changed.     We  can   therefore  let 

1  See  previous  footnote. 


88  THE  GAS  LAWS 

stand  for  the  present  volume  and  correct  it  further  for  pres- 
sure as  follows  :  45  x  273 

293          760 


x  740 

Multiplying  means  and  extremes,  we  have 

76Q  x  =  45  X  273  X  740 

293 

and  dividing  through  by  760,  we  have 
=  45  X  273  X  740 

293  X  760 

The  problem  is  now  in  form  for  numerical  solution.  The 
use  of  logarithms  or  of  the  slide  rule  will  be  of  great  assistance 
in  performing  the  indicated  operations  at  this  point.1  If  the 
pupils  are  not  acquainted  with  logarithms  or  the  slide  rule, 
the  longer  arithmetical  method  will  have  to  be  followed. 

Performing  the  indicated  operation,  x  =  40.8;  thus  we 
see  that  the  volume  would  be  40.8  c.c.  at  standard  conditions. 
Throughout  this  chapter  it  has  been  assumed  that  the 
levels  of  the  liquid  over  which  the  gas  has  been  collected  are 
alike  inside  and  outside  the  gas  measuring  tubes.  In  case 
this  is  not  true,  a  further  correction  must  be  applied  to  the 
calculation  or  else  the  levels  must  be  made  alike.  In  most 
cases,  the  latter  is  the  easier  thing  to  do. 

SUMMARY 

Charles'  Law:  If  the  pressure  remains  constant,  the  volume  of 
any  gas  varies  directly  as  the  absolute  temperature.  In  other 
words,  the  first  volume  is  to  the  final  volume  as  the  first  tem- 
perature is  to  the  final  temperature,  or 

first  volume  _  first  temperature 
final  volume       final  temperature 

1  For  table  of  logarithms,  see  Appendix. 


QUESTIONS  89 

whence,  final  volume  =  first  volume  x  ^nal  temPerature 

first  temperature 

Boyle's  Law:  If  the  temperature  remains  constant,  the  volume  of 
any  gas  varies  inversely  as  the  pressure.  In  other  words, 
the  first  volume  is  to  the  final  volume  as  the  final  pressure  is 
to  the  first  pressure,  or 

first  volume  _  final  pressure 
final  volume      first  pressure 

whence,  final  volume  =  first  volume  x  fi— rpressure 

final  pressure 

Combined  Laws:  When  both  the  temperature  and  pressure  are 
varied  at  the  same  time,  the  combined  laws  may  be  expressed 
as  follows : 

final  volume  =  first  volume  X  ^temperature  x  first  pressure 

first  temperature       final  pressure 


Questions 

What  will  be  the  final  volume  if  25  c.c.  of  gas  at  20°  C.  is 
30°  C.  at  constant  pressure?      j*/tA-£4-4-<c-^ 

2.  If  50.0  c.c.  of  gas  at  30°  C.  is^coolgdJo  20°  C.? 

3.  If  45.0  c.c.  of  gas  at  24°  C.  is  cooled  to  0°  C.? 

4.  If  50.0  c.c.  of  gas  at  -  10°  C.  is  warmed  to  0°  C.? 

5.  If  100.0  c.c.  of  gas  at  0°  C.  is  heated  to  546°  absolute? 

6.  The   temperature   remaining   constant,    calculate   the   final 
volume  if  the  pressure  on  200  c.c.  of  gas  at  740  mm.  is  changed  to 
760  mm. 

7.  If  the  pressure  on  100.0  c.c.  of  gas  at  780  mm.  is  changed  to 
760  mm. 

8.  If  the  pressure  on  40.1  c.c.  of  gas  at  740  mm.  is  changed  to 
780  mm. 

9.  If  the  pressure  on  20  c.c.  of  gas  at  780  mm.  is  changed  to 
740  mm. 

10.   What  will  be  the  final  volume  if  100  c.c.  of  gas  at  760  mm. 
are  compressed  under  1520  mm.  pressure? 


90  THE  GAS  LAWS 

11.  A  given  amount  of  gas  measures  100  c.c.  at  20°  C.  and  740 
mm.     What  will  be  the  volume  under  standard  conditions,  that  is, 
at  0°  C.  and  760  mm.? 

12.  Correct  the  volume  of  50  c.c.  of  gas  at  30°  C.  and  745  mm. 
to  standard  conditions. 

13.  Correct  the  volume  of  55  c.c.  of  gas  at  —  10°  C.  and  780  mm. 
to  standard  conditions. 

14.  A  given  amount  of  gas  measures  55  c.c.  at  —  10°  C.  and  780 
mm.     What  is  its  volume  at  10°  C.  and  740  mm.? 

15.  A  certain  volume  of  gas  at  0°  C.  and  760  mm.  has  its  tem- 
perature raised  until  its  volume  is  doubled,  and  then  the  pressure 
raised  until  its  volume  is  brought  back  to  the  original  value.     What 
are  the  final  conditions  of  temperature  and  pressure? 


CHAPTER  IX 

WATER 

90.  Properties  of  Water.  Water  is  universally  known  as  a 
colorless,  odorless,  tasteless,  mobile  liquid.  There  are  a 
great  many  liquids  besides  water,  however,  and  many  of 
them  are  colorless,  many  are  odorless,  and  some  are  tasteless. 
So  it  appears  that  even  so  common  a  substance  as  water 
might  -be  confused  with  some  other  substance  unless  its 
properties  were  accurately  known.  The  statement  that  has 
already  been  made  applies  well  here  :  that  the  properties  of  a 
substance  are  absolutely  constant.  Hence,  once  knowing 
accurately  the  properties  of  water,  we  may  be  certain  that 
any  liquid  that  has  exactly  the  same  properties  is  also  water, 
but  that  any  liquid  that  shows  any  properties  different  from 
those  of  water  is  not  water. 

It  would  take  a  good  many  pages  to  describe  fully  all  of  the 
properties  of  water,  and  we  may  content  ourselves  with  a 
few  of  the  most  striking  ones  which  are  most  useful  in  dis- 
tinguishing it  from  other  substances. 

Water  is  a  liquid  at  ordinary  temperatures.  At  a  cer- 
tain definite  temperature,  0°  C.,  it  solidifies  to  a  form  that 
we  know  as  ice ;  at  100°  C.,  at  the  ordinary  atmospheric 
pressure,  it  boils,  whereby  it  is  converted  into  a  gas  that  fills 
many  times  the  volume  (in  round  numbers  1700  times) 
that  is  occupied  by  the  water  as  a  liquid.  This  gas  is  known 
as  steam  ;  it  is  colorless  and  transparent,  but  if  it  escapes  into 
the  cold  air,  it  condenses  into  droplets  of  liquid  water  which 
produce  a  cloud  or  mist. 

91 


92  WATER 

91.  Water  as  a  Solvent.     Water  has  the  power  of  dis- 
solving many  substances ;  for  example,  a  substance  like  sugar 
when  put  into  water  goes  into  the  water  and  mixes  uniformly 
with  it,  thereby  becoming  a  part  of  the  liquid,  which  is  now 
a  solution  of  sugar  in  water.     This  power  of  water  to  dis- 
solve other  substances  renders  it  useful  for  cleaning  purposes. 
It  is  also  of  supreme  importance  in  connection  with  the  diges- 
tion and  assimilation  of  food  in  the  body.     Most  chemical 
changes  in  the  practical  work  of  the  manufacturing  chemist, 
as  well  as  most  of  those  that  we  meet  with  in  the  laboratory, 
take  place  in  water  solution.     Hence  the  solvent  power  of 
water  is  of  great  importance. 

Limit  of  Solubility.  On  testing  the  power  of  water  to  dis- 
solve different  substances,  it  is  found  that  there  is  a  definite 
limit  to  the  amount  of  a  substance  that  a  given  amount  of 
water  will  dissolve  at  a  given  temperature.  This  limit  is 
different  for  each  different  substance.  For  example,  100 
grams  of  water  at  25°  C.  will  dissolve  37  grams  of  saltpeter 
(potassium  nitrate),  35  grams  of  common  salt,  and  only  5 
grams  of  boric  acid.  There  is  a  wide  range  of  solubility 
among  the  many  different  substances  that  will  dissolve  in 
water. 

Change  in  Solubility  with  Change  in  Temperature.  In 
general,  the  higher  the  temperature  the  greater  the  solubility 
of  solid  substances  in  water.  Thus  100  grams  of  water  at 
100°  C.  will  dissolve  246  grams  of  saltpeter,  40  grams  of  salt, 
and  27  grams  of  boric  acid.  It  will  be  noted  that  common 
salt  gains  but  little  in  its  solubility  at  100°  C.  over  that  at 
25°  C.  A  few  substances,  among  them  calcium  hydroxide 
(slaked  lime),  become  less  soluble  as  the  temperature  increases. 

92.  Water  in  Chemical  Life  Processes.     Its  power  to  dis- 
solve other  substances  makes  water  essential  in  the  chemistry 


ATMOSPHERIC   MOISTURE  93 

of  the  life  processes  of  plants  and  animals.  Food  in  the 
course  of  digestion  is  transformed  into  soluble  compounds 
wAich  pass  in  solution  through  the  membranes  of  the  di- 
gestive tract  into  the  blood,  and  by  the  blood  are  carried  to 
the  various  parts  of  the  body,  where  they  undergo  further 
reaction  in  the  formation  of  muscular  tissue,  bone,  and  other 
tissues. 

Water  plays  an  equally  important  part  in  plant  life. 
Nitrogen-containing  substances  and  mineral  substances  neces- 
sary for  plant  nutrition  are  in  solution  in  the  water  in  the  soil 
and  are  sucked  up  by  the  roots  of  plants.  Plants,  to  be  sure, 
draw  a  considerable  part  of  their  nutrition  from  the  carbon 
dioxide  of  the  atmosphere.  This  reacts  with  water  in  the 
cells  of  the  green  leaves  and  forms  sugar-like  substances  which 
are  conveyed  in  the  sap  to  the  growing  parts  of  the  plant. 
Thus  water  is  absolutely  essential  to  all  the  chemical  processes 
taking  place  in  animals  and  plants 

93.  Irrigation.     Some  of  the  deserts  in  our  great  West 
which  were  formerly  absolutely  barren  of  any  useful  vegeta- 
tion are  now,  through  irrigation,  among  the  most  fertile  lands 
of  the  country.     The  necessary  food  materials  were  in  the  soil, 
but  lacked  the  solvent  power  of  water  to  make  them  available 
for  plant  life. 

94.  Atmospheric  Moisture,  Humidity,  and  Precipitation. 
The  greater  part  of  the  cultivated  land  depends  upon  the 
rain  for  its  moisture.     Even  when  the  land  is  irrigated,  the 
water  comes  from  rivers  which  are  supplied  by  the  rain  and 
snow  that  fall  on  the  mountains.     So  the  ability  of  the  atmos- 
phere to  contain  water  vapor  is  essential  to  the  existence  of 
life. 

It  is  a  well-known  fact  that  wet  clothes  are  hung  out  to 
dry.  The  water  evaporates  and  becomes  a  part  of  the  air. 


94  WATER 

Likewise  the  water  of  the  ocean  evaporates  and  the  moisture 
is  carried  by  the  wind  over  the  land.  Now  the  amount  of 
water  which  can  evaporate  into  a  given  space  depends  only 
on  the  temperature  and  varies  very  greatly  with  change  in 
temperature.  At  0°  C.  one  cubic  meter  (about  1.3  cubic 
yards)  of  space  can  take  up  nearly  5  grams  of  water  vapor ; 
at  20°  C.  or  approximately  room  temperature,  17  grams,  at 
30°  C.  or  warm  summer  temperature,  30  grams ;  at  40°  C.  or 
excessively  warm  summer  temperature,  52  grams. 

Relative  humidity  is  a  term  used  in  reporting  atmos- 
pheric conditions.  When  the  air  contains  the  maximum 
possible  water  vapor  at  the  given  temperature,  it  is  saturated 
and  its  relative  humidity  is  100  per  cent.  With  lesser 
amounts  of  moisture,  it  is  unsaturated  and  its  humidity  is 
expressed  in  per  cent  of  the  saturation  value  for  that  tem- 
perature. 

Precipitation  occurs  when  the  air  becomes  supersaturated 
with  moisture.  This  can  occur  when  nearly  saturated  warm  air 
becomes  cooled.  Its  capacity  for  water  vapor  is  much  lower 
at  the  lower  temperature  and  so  what  will  only  saturate  at  the 
higher  temperature  will  more  than  saturate  the  air  at  the 
lower  temperature.  Therefore  the  excess  above  the  satura- 
tion point  precipitates  as  mist  or  rain.  Water  vapor  is  entirely 
invisible.  For  example,  the  average  schoolroom  contains 
one  kilogram  or  about  two  pounds  of  water  as  invisible 
vapor.  Likewise,  the  air  on  a  perfectly  clear  day  may 
contain  large  amounts  of  water  vapor.  If  the  air  currents 
become  cooled,  mist  and  clouds  are  formed.  Mist  consists 
of  a  vast  number  of  very  small  globules  of  liquid  water, 
which  are  so  small  that  they  remain  suspended  in  the  atmos- 
phere. If  these  mist  globules  become  larger,  they  fall  to  the 
earth  as  rain. 


DRINKING  WATER  95 

95.  Relative  Humidity  and  Human  Comfort.    The  water 
vapor  is  a  very  important  constituent  of  the  air  when  con- 
sidered from  the  standpoint  of  human  comfort.     When  the 
relative  humidity  is  high,  human  beings  become  very  uncom- 
fortable because  the  perspiration  fails  to  evaporate  promptly 
from  the  surface  of  the  body.     Now  the  evaporation  of  per- 
spiration is  one  of  the  most  important  means  of  regulating  the 
body  temperature,  heat  being  taken  up  when  the  moisture 
evaporates.     Hence,  when  the  relative  humidity  is  high,  peo- 
ple are  usually  very  uncomfortable.     High  relative  humid- 
ity is  more  oppressive  than  a  high  temperature  accompanied 
by  a  relatively  dry  atmosphere.     The  oppressive  character 
of  the  air  in    schoolrooms,  crowded  halls,  or    other  public 
gathering  places  is  largely  due  to  the  high  relative  humidity 
caused  by  the  evaporation  of  perspiration  and  by  the  water 
vapor  exhaled  with  the  air  from  the  lungs. 

96.  Drinking  Water.     Those  who  have  always  lived  in 
cities  and  have  had  an  abundant  supply  of  clear,  pure  water 
whenever  they  chose  to  turn  the  faucet,  find  it  hard  to  realize 
what  a  serious  matter  the  problem  of  water  supply  is.     Water 
that  is  to  be  used  for  drinking  must  be  pure  from  a  sani- 
tary standpoint,  that  is,  it  must  contain  nothing  that  can 
cause  sickness.     Bacteria,  —  microscopic  living  organisms, — • 
which  cause  certain  diseases,   notably  typhoid  fever,   are 
likely  to  be  present  in  water  as  a  result  of  pollution  by 
sewage.     . 

A  few  bacteria  taken  into  the  system  in  a  glass  of  even 
badly  polluted  water  are  not  in  themselves  harmful,  but  the 
danger  lies  in  the  fact  that  they  find  in  the  human  intestines  a 
favorable  place  to  multiply.  So,  if  a  few  disease  germs  get 
into  the  system,  they  may  within  a  short  time  have  increased 
until  there  are  countless  numbers  of  them.  Usually  the  hu- 

B.  AND  W.  CHEM. 7 


96  WATER 

man  system  is  strong  enough  to  destroy  the  disease  germs 
before  they  gain  the  ascendency.  But  when  the  system  is 
weakened  through  abuse,  or  overwork,  or  lack  of  exercise,  it 
falls  an  easy  prey  to  disease  bacteria. 

The  best  safeguard  against  disease  is  to  keep  the  body  in 
healthy  condition,  but  still  one  will  not  willingly  subject  it  to 
infection  by  disease  germs.  Hence  those  in  charge  of  city 
water  supplies  must  be  continually  on  the  alert  to  keep  them 
free  from  infection.  The  appearance  is  not  a  criterion  of  the 
safety  of  water.  The  clearest  looking  water  may  contain 
millions  of  typhoid  bacilli,  whereas,  often,  yellow  and  even 
muddy  water  may  be  quite  safe.  It  requires  chemical  and 
bacteriological  tests  to  prove  the  danger  or  safety  of  water, 
and  in  city  supplies  the  city  chemists  and  bacteriologists 
must  be  depended  upon  to  see  to  this.  In  country  places, 
where  each  house  has  its  own  well  or  spring,  the  safety  of  the 
water  should  be  determined  from  time  to  time  by  analysis. 
Samples  of  water  are  usually  tested  free  of  charge  at  the 
laboratories  of  the  state  board  of  health.  This  testing  is 
particularly  necessary  when  the  well  is  near  the  closet  or  barn- 
yard, but  distance  from  apparent  source  of  infection  cannot 
always  be  taken  as  assurance  of  safety. 

97.  The  purification  of  water  for  sanitary  purposes 
consists  in  the  main  in  destroying  or  removing  bacteria. 
Filtration  will  accomplish  the  removal  if  the  pores  of  the 
filter  are  small  enough.  The  ordinary  filter  which  screws  on 
the  faucet  is  entirely  valueless  for  this  purpose.  The  type 
of  filter  which  is  made  of  unglazed  porcelain  is  effective  in 
removing  bacteria,  but  its  pores  are  so  small  that  the  water 
filters  very  slowly. 

Sand  filters  are  used  in  purifying  large  city  supplies  but  the 
safety  of  these  filters  is  due  in  part  to  the  action  of  beneficent 


PURIFICATION  OF  WATER  97 

bacteria  which  abound  in  the  filter  and  destroy  the  harmful 
organisms. 

Chemical  destruction  of  bacteria  is  a  great  safeguard. 
When  water  is  stored  for  a  long  period  in  reservoirs,  the 
bacteria  are  destroyed  through  the  oxidizing  action  of  the 
air.  More  rapid  destruction  may  be  accomplished  through 
the  use  of  chlorine  or  bleaching  powder,  and  recently  the  use 
of  ozone,  a  particularly  active  modification  of  oxygen  (see 
Chapter  XV,  p.  159)  has  come  into  favor,  particularly  in 
European  cities. 

Sterilization  by  Boiling.  In  country  places  which  have  to 
depend  on  polluted  wells  and  in  cases  of  epidemic  in  cities 
when  the  public  health  authorities  cannot  control  the  situa- 
tion, danger  of  infection  is  obviated  by  boiling  the  water  for 
at  least  five  minutes,  since  none  of  the  disease-producing 
organisms  are  capable  of  withstanding  the  boiling  tempera- 
ture for  that  length  of  time. 

98.  Chemically  Pure  Water.     Water  that  is  pure  from  a 
sanitary  standpoint  may  contain  considerable  quantities  of 
dissolved    mineral    matter.      Indeed,  water  with    a  small 
mineral  content  is  probably  more  desirable  for  drinking  than 
absolutely  pure  water. 

Natural  waters  may  be  purified  of  their  mineral  content  by 
means  of  distillation  (see  page  17).  Rain  water  has  been 
subjected  to  a  natural  process  of  distillation  and  is  free  from 
mineral  matter  except  as  it  drags  down  smoke  and  dust  from 
the  atmosphere. 

99.  Water  for  industrial  purposes,  as,  for  example,  for  use  in 
laundries  or  for  feeding  steam  boilers,  should  contain  as  small 
a  mineral  content  as  possible  (see  Hard  Water,  page  201). 
It  is,  of  course,  impossible  from  the  standpoint  of  expense  to 
distil  all  the  water  needed  in  industrial  operations,  and  it  is 


98  WATER 

therefore  very  necessary  to  obtain  a  natural  supply  as  free  as 
possible  from  dissolved  minerals. 

SUMMARY 

Water  is  a  colorless,  odorless,  tasteless,  mobile  liquid  which  freezes 
at  0°  C.  and  boils  at  100°  C.  One  volume  of  liquid  water 
gives  about  1700  volumes  of  steam. 

Water  is  a  solvent  for  a  great  many  substances.  Each  substance 
has  its  own  definite  limit  of  solubility.  In  general  solid  sub- 
stances are  more  soluble  the  higher  the  temperature.  The 
solvent  power  of  water  makes  it  very  useful  for  cleansing  pur- 
poses and  essential  in  the  life  processes  of  plants  and  animals. 

Water  vapor  in  the  atmosphere  is  the  source  of  rain  and  of  all  natural 
fresh  water.  Precipitation  occurs  when  the  atmosphere  is 
left  more  than  saturated  with  water  vapor  as  a  result  of 
cooling. 

In  arid  regions,  the  conveyance  of  water  from  a  distance  permits 
the  raising  of  abundant  crops. 

Relative  humidity  is  expressed  in  per  cent  of  the  moisture  neces- 
sary for  saturation  at  the  given  temperature. 
Human  comfort  depends  largely  on  the  degree  of  relative  hu- 
midity. 

Drinking  water  is  often  rendered  dangerous  by  the  presence  of 
disease-producing  bacteria,  so  that  wells  and  other  water 
supplies  should  be  tested. 

Impure  water  may  be  rendered  safe  by  proper  filtration  or  by 
sterilization.     Water  may  be  sterilized  by  boiling  it  for  at 
least  five  minutes  or  by  the  addition  of  chemical  substances 
such  as  chlorine  or  ozone. 
Water  may  be  freed  of  mineral  matter  by  distillation. 

Questions 

1.  How  could  you  make  sure  that  a  certain  colorless  liquid  was 
water  ? 

2.  On  a  cold  day,  with  no  tendency  for  ice  to  melt,  how  could 
you  distinguish  ice  from  rock  crystal? 


QUESTIONS  99 

3.  Cite  some  experiments  by  which  you  can  prove  that  the  air 
of  the  schoolroom  contains  water  vapor. 

4.  Why  is  it  necessary,  from  a  chemical  standpoint,  for  us  to 
drink  water  ? 

5.  Some  of  our  western  deserts  are  very  fertile  when  irrigated. 
Would  they  be  equally  fertile  to-day  if  they  had  been  subjected  to 
rainfall  for  the  past  century? 

6.  Explain  the  cause  of  the  dew  found  on  the  grass  in  the 
morning. 

7.  Why  are  people  so  uncomfortable  in  humid  weather  ? 

8.  State  and  explain  the  necessary  precautions  to  be  taken  in 
connection  with  the  source  of  your  drinking  water. 

9.  How  are  municipal  water  supplies  safeguarded? 

10.  Why  do  not  the  same  standards  of  purity  apply  to  water 
for  drinking  and  for  industrial  purposes? 

11.  Supposing  the  fresh  water  supply  had  given  out,  how  might 
drinking  water  be  obtained  in  mid-ocean  on  a  steamship? 


CHAPTER  X 

COMPOSITION  OF  WATER 

IN  preceding  chapters  we  have  seen  that  although  air  was 
regarded  by  the  ancients  as  one  of  the  elements,  it  consists  in 
reality  of  two  different  gases,  oxygen  and  nitrogen,  not  to 
mention  small  quantities  of  several  other  gases.  Water 
was  regarded  by  the  ancients  as  another  of  the  elements, 
but  we  are  to  show  that  it  is  in  reality  a  compound  of  two 
elements,  oxygen  and  hydrogen ;  for  it  can,  by  suitable  means, 
be  decomposed  into  these  two  elements. 

100.  Decomposition  of  Water  by  Hot  Iron.  For  ex- 
ample, by  boiling  water  and  leading  the  steam  through  a 
tube  containing  red-hot  iron  filings,  we  find  that  the  vapor 
which  escapes  from  the  farther  end  of  the  tube  does  not 
entirely  condense  again  to  the  liquid  water,  as  it  would  if 
no  chemical  change  had  taken  place,  but  that  a  part  of  it 
remains  permanently  as  a  gas,  Fig.  18.  This  gas  does  not 
dissolve  in  water;  it  is  colorless  and  odorless;  it  burns 
readily  but  it  does  not  itself  support  combustion ;  in  short, 
it  is  an  entirely  new  substance,  different  in  its  properties  from 
either  the  iron  or  the  water  used  in  obtaining  it.  It  is 
hydrogen.  This  gas  cannot  have  come  from  the  iron,  for 
in  all  our  experience  with  iron  no  other  substance  has  ever 
been  obtained  from  it,  that  is,  iron  is  an  element.  The  gas 
must,  then,  have  come  from  the  only  other  possible  source ; 
that  is,  from  the  water. 

100 


DECOMPOSITION   OF  WATER*  3t  'HOT  IRON 


101 


If  we  examine  the  substance  remaining  in  the  tube,  we 
find,  in  place  of  the  iron,  a  black  substance  which  is  brittle, 
rather  than  malleable  like  iron.  This  substance  is  oxide  of 


iron. 


The  water,  therefore,  must  have  had  in  it  not  only  hydrogen, 
but  also  something  which  united  with  the  element  iron  to 


FIG.  18.  —  Decomposition  of  Steam  by  Hot  Iron. 

form  the  iron  oxide.  Upon  studying  the  properties  of  this 
iron  oxide  more  completely,  we  find  it  to  be  the  same  sub- 
stance that  was  obtained  when  iron  was  burned  in  pure 
oxygen  (see  page  44).  As  the  same  elements  are  required 
to  produce  the  same  chemical  substance  in  every  case,  we 
thus  have  made  certain  that  there  must  be  some  oxygen  in 
this  new  substance  formed  from  iron  and  water.  Since 
iron  is  an  element,  the  oxygen  must  have  come  from  the 


102  COMPOSITION   OF  WATER 

water,  and  water  must  therefore  contain  oxygen  as  well  as 
hydrogen. 

101.  Decomposition  of  Water  by  Sodium.  Many  other 
metals  when  hot  can  decompose  water  vapor.  Hydro- 
gen is  liberated  because  the  metal  attaches  the  oxygen  to 
itself  in  each  case  and  thus  leaves  nothing  to  hold  the  hydro- 
gen in  combination.  Some  metals  are  sufficiently  active 
towards  water  to  decompose  it  even  when  cold;  but  such 
metals  are  never  found  free  in  nature,  as  they  would  long 
ago  have  been  acted  upon  by  natural  waters.  These  very 
active  metals  can  be  prepared  by  decomposing  such  of  their 
compounds  as  are  readily  available,  and  this  is  most  often 
accomplished  by  the  aid  of  the  electric  current.  One  of 
these  metals  is  sodium,  which  is  prepared  in  considerable 
quantities  by  decomposing  sodium  hydroxide  by  means  of 
the  electric  current. 

If  a  piece  of  sodium,  which  is  lighter  than  water,  is  thrown 
upon  water,  a  lively  action  begins  at  once ;  a  gas  is  evolved, 
and  the  bit  of  sodium  darts  round  and  round  the  dish,  due 
to  the  reaction  from  the  pressure  of  the  issuing  gas.  The 
sodium  melts  from  the  heat  evolved  by  the  chemical  reaction 
and  then  forms  a  round  ball  which  speedily  grows  less  and 
less  in  size  and  then  disappears.  If  the  reaction  is  carried 
out  under  an  inverted  cylinder  full  of  water,  the  gas  evolved 
rises  to  the  top  of  the  cylinder.1  The  gas  can  then  be  ex- 
amined and  its  properties  determined.  It  proves  to  be 
hydrogen.  Thus  sodium  can  accomplish,  in  the  cold, 
what  iron  could  only  accomplish  when  the  temperature 
was  raised;  namely,  the  displacement  of  hydrogen  from 
water. 

1  This  experiment  should  be  performed  only  by  the  instructor,  as 
an  explosion  may  result  if  not  carefully  handled. 


ELECTROLYSIS  OF  WATER  103 

102.  Further  Displacement  of  Hydrogen.     If,  now,  the 
solution  remaining  after  the  action  of  sodium  on  water  is 
evaporated  to  dry  ness,  thus  removing  all  water  not  acted 
upon  by  the  sodium,  the  product  is  found  to  be  a  white  solid 
which  has  all  the  properties  of  the  substance  known  as 
sodium  hydroxide.     On  powdering  this  product  and  mixing 
it  with  an  equal  bulk  of  powdered  zinc  and  heating  the 
mixture  gently  in  a  dry  test  tube,  a  quantity  of  gas  is  evolved 
which  on  testing  proves  to  be  hydrogen.     Now  both  sodium 
and  zinc  are  known  to  be  elements  and  therefore  do  not  con- 
tain hydrogen.     The  hydrogen,  then,  since  it  cannot  have 
come  from  either  of  the  metals  employed,  must  have  come 
originally  from  the  water.     Thus  we  have  twice  obtained 
hydrogen,  which  was  originally  a  component  of  the  same  water, 
and  we  can  assert  that  the  hydrogen  of  water  exists  in  two 
portions,  one   of  which  is   more   easily  displaced  than  the 
other.     An  accurate  comparison  of  the  amounts  of  hydrogen 
set  free  by  the  sodium  and  by  the  zinc,  respectively,  has 
shown  that  they  are  equal. 

103.  Electrolysis  of  Water;    Volume  Composition.    Our 
next  step  in  the  study  of  water  will  be  to  study  the  effect  of 
passing  an  electric  current  through  it.     An  electric  current 
passes  through  pure  water  with  great  difficulty,  but  if  a  little 
sulphuric  acid  is  added  to  the  water,  the  current  then  passes 
freely.     The  exact  nature  of  the  action  of  the  sulphuric  acid 
in  allowing  the  current  to  flow  more  freely  will  not  be  ex- 
plained here,  but  it  may  be  stated  that  there  is  just  the  same 
amount  of  acid  present  after  the  passage  of  a  large  amount 
of  current  as  there  was  at  first.     In  this  experiment,  then, 
we  shall  add  a  little  sulphuric  acid  to  the  water  as  a  matter 
of  convenience ;  but  in  studying  the  changes  that  occur,  we 
shall  leave  the  acid  entirely  out  of  account,  for  as  just  stated 


104 


COMPOSITION  OF  WATER 


the  acid  itself  is  not  permanently  altered.  The  electrolysis  of 
water  can  best  be  carried  out  in  an  apparatus  devised  by 
Hofmann  and  known  as  Hofmann's  electrolytic  apparatus. 
It  consists  of  three  tubes  connecting  at  the  bottom,  as  shown 
in  Fig.  19.  Two  of  the  tubes  are  in  the  same  plane,  the 
third  is  in  the  rear  and  is  provided 
with  a  bulb  at  the  top  to  act  as  a 
reservoir  for  the  acidulated  water. 
In  the  lower  part  of  the  front  tubes 
are  placed  small  pieces  of  sheet 
platinum  which  are  supported  by 
platinum  wires  connected  respec- 
tively with  the  two  poles  of  an  elec- 
tric battery.  These  metal  'terminals 
are  known  as  electrodes  and  it  is  at 
the  surface  of  these  electrodes  that 
the  chemical  changes  brought  about 
by  the  electric  current  take  place. 
The  tops  of  the  two  front  tubes 
are  provided  with  glass  stopcocks 
through  which  any  gases  that  collect 
may  be  removed  for  examination. 

On  filling  the  apparatus  with  the 
acidulated  water  and  allowing  the 
electric  current  to  pass,  bubbles  of 
gas  at  once  appear  at  each  of  the 

electrodes  and  continue  to  rise  so  long  as  the  current  is 
passing.  It  is  soon  evident  that  more  gas  collects  at  the 
negative  electrode  (that  is,  the  one  connected  with  the  nega- 
tive electrical  conductor)  than  at  the  positive  electrode, 
and  when  carefully  measured  under  like  conditions  of 
temperature  and  pressure,  it  is  found  that  the  volume  of 


FIG.   19. — Electrolysis  of 
Water. 


SYNTHESIS  OF   WATER  105 

gas  collected  at  the  negative  electrode  is  exactly  twice  that 
collected  at  the  positive.  On  withdrawing  some  of  the  gas 
that  occupies  the  larger  volume  and  testing  it,  we  find  that 
it  will  burn,  but  will  not  support  combustion ;  it  is  colorless 
and  odorless,  and  insoluble  in  water.  It  has,  in  fact,  the 
characteristic  properties  of  hydrogen.  The  other  gas,  that 
from  around  the  positive  electrode,  is  colorless  and  odorless, 
it  will  not  burn,  but  it  does  support  combustion ;  it  shows 
the  characteristic  properties  of  oxygen. 

104.  Synthesis  of  Water.     After  showing  thus  that  water 
can  be  decomposed  into  two  volumes  of  hydrogen  gas  and 
one  volume  of  oxygen  gas,  the  thought  naturally  arises :  if 
this  is  all  that  water  consists  of,  it  should  be  possible  to  form 
water  from  these  two  gases  when  they  are  taken  in  the  pro- 
portion of  two  volumes  of  the  former  to  one  volume  of  the 
latter.     And,  as  a  matter  of  fact,  this  can  be  done  with 
perfect  ease  for  there  is  great  chemical  attraction  between 
hydrogen  and  oxygen  (it  is  this  attraction  for  each  other  that 
has  to  be  overcome  when  water  is  decomposed  by  the  electric 
current  or  other  means) .     To  form  water,  it  is  only  necessary 
to  bring  hydrogen  and  oxygen  into  the  presence  of  each  other 
and  to  start  chemical  combination  by  means  of  a  spark  or  a 
small  flame.     For  example,  hydrogen  gas  issuing  from  a  jet 
may  be  lighted  just  as  ordinary  illuminating  gas  is  lighted. 
The    hydrogen    burns,    however,   with    a    colorless    flame. 
That  water   is   really  a  product  of  this   reaction   may  be 
proved  by  holding  a  piece  of  cold  glass  over  the  flame  and 
noting  the  moisture  that  condenses. 

105.  By  means   of   the   apparatus   shown   in  Fig.  20,  it 
can  be  readily  shown  that  two  volumes  of  hydrogen  and 
one  volume  of  oxygen  actually  do  combine  to  form  water. 
The  inner  tube  is  filled  to  mark  1  with  oxygen  and  then  to 


106 


COMPOSITION  OF  WATER 


mark  3  with  hydrogen,  precaution  being  taken  by  raising  or 
lowering  the  apparatus  that  the  top  of  the  mercury  column 
in  the  tube  is  always  at  the  same 
height  above  the  free  surface  of 
mercury  in  the  reservoir  M.  After 
causing  combination  to  take  place  by 
jumping  a  spark  between  the  term- 
inals of  the  electrical  conductors, 
E  E,  we  see  the  mercury  almost 
immediately  rise  until  it  reaches 
the  top  of  the  tube,  if  the  experi- 
ment is  carried  out  at  the  ordinary 
temperature. 

When  however  the  experiment  is 
performed  while  steam  is  being  passed 
through  the  outer  jacket,  thus  keep- 
ing the  temperature  constantly  at 
100°  C.,  the  mercury  level  rises  only 
to  the  mark  2  (the  same  precaution 
being  observed  of  adjusting  the  ap- 
paratus until  the  top  of  the  mercury 
is  at  the  same  height  above  the  level 
in  the  reservoir). 

We   have   thus   shown  that  at  a 
temperature  where  water  will  remain 
a  gas,  two  volumes  of  hydrogen  com- 
bine with  one  volume  of  oxygen  to 
produce  two  volumes  of  water  vapor. 
106.   Law   of   Definite  Combining 
FIG.  20. -combining  Pro-  Proportions  Illustrated.     These  gases 
portions  by  Volume  of  Hydrogen  WJU  not  combine  in  any  other  pro- 
portions than   those  stated.     If,   for 


\M 


WEIGHT   COMPOSITION  OF  WATER  107 

example,  one  volume  of  oxygen  is  mixed  with  three  volumes 
of  hydrogen,  two  volumes  of  the  latter  combine  as  before 
to  produce  water,  while  one  volume  is  left  uncombined  and 
will  remain  as  a  permanent  gas  even  if  the  tube  is  cold  and 
the  water  vapor  condenses  to  liquid  water.  Similarly,  if 
two  volumes  of  oxygen  are  mixed  with  two  volumes  of 
hydrogen,  it  is  found,  after  the  explosion,  that  one  volume 
of  oxygen  is  left  uncombined.  These  facts  illustrate  again 
the  law  of  definite  proportions. 

Such  simple  and  exact  relations  as  are  thus  observed  in 
the  combining  volumes  of  these  gases  would  never  exist  by 
mere  chance.  There  must  be  some  underlying  explanation 
for  it  and  we  shall  try  in  a  forthcoming  chapter  to  develop 
such  an  explanation. 

107.  Weight  Composition  of  Water.  Very  exact  measure- 
ments of  the  weights  of  these  gases  have  shown  that  one 
liter  of  oxygen  at  0°  C.  and  760  mm.  pressure  weighs 
lI42°/_^g.,  while  one  liter  of  hydrogen  weighs  0.08996  g. 
Since,  as  we  have  seen,  two  volumes  of  hydrogen  com- 
bine with  one  of  oxygen,  it  follows  that  1.429  g.  of 
oxygen  combine  with  0.17992  g.  of  hydrogen.  These 
weights  are  in  the  porportion  of  7.94  to  1.00;  in  other  words, 
7.94  parts  by  weight  of  oxygen  combine  with  one  part  by 
weight  of  hydrogen  to  produce  8.94  parts  by  weight  of  water. 
The  combining  weight  of  oxygen  is,  then,  7.94,  if  that  of 
hydrogen  is  considered  as  one. 

We  shall  find  in  every  case  in  which  two  elements  com- 
bine, as  do  hydrogen  and  oxygen  in  this  case,  to  form  a 
definite  chemical  compound  such  as  water,  that  there  is  a 
certain  definite  ratio  between  the  weights  of  the  two  ele- 
ments that  enter  the  combination.  Although  these  ratios  are 
always  absolutely  the  same  for  the  same  combination,  they 


108  COMPOSITION  OF   WATER 

are  not  as  a  rule  simple  whole  numbers.  Contrasted 
with  this,  the  ratios  of  the  volumes  of  gaseous  substances 
which  combine  are  not  only  absolutely  definite,  but  they 
are  in  the  proportion  of  simple  whole  numbers.  We  shall 
seek  for  explanations  of  these  interesting  facts  in  a  future 
chapter. 

108.  Hydrogen  peroxide  is  a  compound  that,  like  water, 
contains  only  the  elements  hydrogen  and  oxygen,  but  here 
we  find  that  15.88  parts  by  weight  of  oxygen  instead  of  7.94 
parts  are  combined  with  one  part  of  hydrogen. 

The  common  pharmaceutical  preparation  which  is  some- 
times sold  under  the  correct  name  of  hydrogen  peroxide  and 
sometimes  under  trade  names  suggesting  the  double  amount 
of  oxygen,  is  a  mixture  of  about  3  parts  of  hydrogen  peroxide 
with  97  parts  of  water.  It  is  used  in  cleansing  wounds  since 
it  has  a  destructive  action  on  the  germs  that  cause  blood 
poisoning,  while  it  is  not  injurious  to  the  flesh. 

Pure  hydrogen  peroxide  is  a  sirupy  liquid  one  and  one  half 
times  as  heavy  as  water.  It  blisters  the  skin  and  will  set  fire 
to  finely  divided  cotton.  The  pure  substance  is  dangerous  to 
handle,  since  it  will  explode  violently  on  rather  slight  prov- 
ocation. The  explosion  is  due  to  a  sudden  decomposition 
into  water  and  oxygen.  In  solution  in  water  it  is  not  danger- 
ous, but  it  decomposes  with  ease,  giving  off  oxygen  and  only 
water  remaining.  If  a  pinch  of  some  powdered  substance, 
sand,  or,  still  better,  manganese  dioxide,  is  dropped  into  a 
3  per  cent  hydrogen  peroxide  solution  in  a  test  tube,  a  vigor- 
ous effervescence  begins  at  once  and  the  escaping  gas  will 
cause  a  glowing  splinter  to  burst  into  flame.  The  powder 
can  be  shown  not  to  have  changed  chemically,  thus  it  acts 
only  catalytically  in  hastening  the  natural  decomposition  of 
the  hydrogen  peroxide. 


SUMMARY  109 

SUMMARY 

Water  is  not  an  element  as  the  ancients  believed.  It  is  composed 
of  the  elements  hydrogen  and  oxygen.  It  may  be  decomposed 
by  iron  at  a  high  temperature  or  by  a  very  active  metal  such 
as  sodium  at  the  ordinary  temperature.  Hydrogen  gas  is 
evolved  in  these  cases  and  a  residue  containing  oxygen  and 
the  metal  is  left. 

Volumetric  Composition:  Decomposition  of  water  by  the  electric 
current  and  synthesis  of  water  from  its  elements  show  that 
two  volumes  of  hydrogen  and  one  volume  of  oxygen  enter 
into  its  composition.  When  the  temperature  is  high  enough 
to  keep  the  water  in  the  gaseous  form,  the  two  volumes  of 
hydrogen  and  one  volume  of  oxygen  yield  two  volumes  of 
water  vapor. 

The  proportion  by  weight  in  which  the  elements  combine  in  water 
are  7.94  parts  of  oxygen  and  1.00  part  of  hydrogen.  This 
relation  is  as  definite  but  not  as  simple  as  the  volume  relation. 

Hydrogen  peroxide  is  another  compound  of  hydrogen  and  oxygen, 
and  for  a  given  Weight  of  hydrogen  it  contains  exactly  twice 
as  much  oxygen  as  does  water. 

Questions 

1.  How  do  you  know  that  the  ancients  were  wrong  when  they 
said  that  water  is  an  element? 

2.  Which  would  give  a  sharper  explosion  —  a  mixture  of  oxygen 
and  hydrogen  in  which  the  two  gases  were  present  in  equal  volumes, 
or  one  containing  two  volumes  of  hydrogen  to  one  of  oxygen? 
Why? 

3.  Which  weighs  more  —  one  liter  of  oxygen  or  two  liters  of 
hydrogen?     How  much  more? 

4.  Thirty  cubic  centimeters  of  hydrogen  and  30  c.c.  of  oxygen 
are  collected  over  mercury  in  a  measuring  tube  at  20°  C.     After 
exploding  the  mixture,  what  will  be  the  volume  at  20°  C.  if  the 
pressure  is  kept  the  same  ? 

6.  If  30  c.c.  of  hydrogen  and  30  c.c.  of  oxygen  are  collected  in  a 
measuring  tube  at  100°  C.  and  the  tube  is  kept  at  this  temperature, 
by  surrounding  it  with  a  steam  jacket,  what  will  be  the  total 


110  COMPOSITION  OF  WATER 

volume  of  gases  present  after  exploding  the  mixture,  if  the  pressure 
is  kept  the  same  ?  What  are  the  gases  ?  What  is  the  volume  of 
each? 

6.  An  evacuated  heavy  glass  tube  is  filled  at  room  temperature 
with  oxygen  until  the  pressure  is  76  cm.  and  then  hydrogen  is 
forced  in  until  the  pressure  has  risen  to  228  cm.     The  mixture 
is  then  exploded  by  a  spark.     What  will  be  the  condition  in  the 
tube  after  it  has  cooled  to  the  original  temperature? 

7.  If  the  same  evacuated  tube  is  kept  at  100°  C.  and  filled  with 
oxygen  to  10  cm.  pressure  and  with  hydrogen  to  50  cm.  pressure, 
what  will  be  the  pressure  after  the  explosion  if  the  temperature  is 
still  100°  C.  ?  —  if  the  tube  is  cooled  to  0°  C.  ?     (Neglect  pressure  of 
water  vapor  at  0°  C.) 

8.  A  certain  substance  yields  equal  volumes  of  hydrogen  and 
chlorine  gases  on  decomposition.     Find  weight  per  liter  of  each  of 
these  gases  in  the  Appendix  (page  428)  and  calculate  the  weight 
composition  of  the  substance. 

9.  Would  a  water  chemist  in  "  analyzing  "  a  sample  of  water 
decompose  the  water  as  described  in  this  chapter?    Would  he  be 
at  all  interested  in  a  real  analysis  of  water  in  such  a  case?    Why? 

10.  In  the  practical  uses  of  water  does  the  water  usually  become 
decomposed? 

11.  Explain  how  hydrogen  peroxide  and  water  illustrate  the  law 
of  multiple  proportions.     What  other  illustration  of  this  law  have 
we  already  encountered? 

12.  Explain  the  action  caused  by  dropping  manganese  dioxide 
into  3  per  cent  hydrogen  peroxide. 

13.  How  does  hydrogen  peroxide  destroy  germs? 

14.  Explain  why  pure  hydrogen  peroxide  will  blister  the  skin 
and  set  fire  to  cotton. 


CHAPTER  XI 
HYDROGEN 

HYDROGEN,  as  has  been  seen  in  the  last  chapter,  is  one  of 
the  constituents  of  water,  and  when  liberated  from  its  com- 
bination with  oxygen  in  that  compound  it  assumes  the  form 
of  a  gas. 

109.  Discovery  of  Hydrogen.     Hydrogen  was  discovered 
in  1766  by  Cavendish,  who  was  the  first  to  make  clear  its 
elementary  character,   but  it  had  been  obtained  prior  to 
Cavendish's  discovery  by  Paracelsus  and  perhaps  by  others 
while  working  with  active  metals  and  sulphuric  acid.     It 
was  not  called  hydrogen  by  Cavendish,  but  by  Lavoisier 
who  named  it  from  Greek  words  meaning  the  water  former, 
because  when  it  is  burned  in  air  or  oxygen,  water  is  formed. 

110.  Hydrogen  in  Water.     Water  is  in  fact  the   most 
abundant   substance  of  which  hydrogen  is  a  constituent. 
In  water,  as  we  have  learned  in  the  last  chapter,  hydrogen 
and  oxygen  are  held  together  by  some  force,  so  that,  instead 
of  having  a  mixture  of  two  gases,  we  have  the  liquid  water, 
a    substance    which    has    different    properties    from    either 
hydrogen  or  oxygen.     By  overcoming  the  force  that  holds 
its  constituents  together  it  should  be  possible  to  separate 
water  again  into  its  elements.     As  already  seen  in  the  last 
chapter,   water   can   be  decomposed  into  its   elements   by 
means  of  the  energy  of  the  electric  current;    and  hydrogen 
may  be  set  free  from  water  by  means  of  some  metals,  as 

B.  AND  W.  CHEM. 8  111 


112 


HYDROGEN 


sodium  or  iron,  which  possess  a  greater  chemical  energy  and 
therefore  a  greater  attraction  for  the  oxygen. 

111.  Hydrogen  from  Acids.     When   hydrogen   is   to  be 
made  in  the  laboratory,  water  is  not  chosen  as  its  source,  but 
rather  an  acid  such  as  sulphuric  acid  or  hydrochloric  acid. 
An  acid  always  contains  hydrogen,  although  not  all  sub- 
stances that  contain  hydrogen  are   acids.     In  an    acid  the 
hydrogen  is  in  such  a  peculiar  state  of  combination  that  if 
the  acid  is  dissolved  in  water  and  the  solution  treated  with 
the  metal  zinc,  or  iron,  or  any  one  of  a  considerable  number 
of  active  metals,  the  hydrogen  is  displaced  by  the  metal, 
escaping  as  a  gas,  while  the  metal  assumes  the  place  in  the 
acid  which  was  vacated  by  the  hydrogen. 

112.  Hydrogen  Generator.     The  simplest  form  of  labora- 
tory generator  consists  of  a  bottle  or  flask  (Fig.  21)  with 


FIG.  21.  —  Hydrogen  Generator. 

a  two-holed  stopper  through  one  hole  of  which  a  thistle 
tube  enters,  reaching  the  bottom  of  the  bottle,  and  thus 
serving  not  only  for  the  entrance  of  fresh  acid  but  also  as  a 


%  PROPERTIES  OF  HYDROGEN  113 

trap  to  prevent  the  escape  of  the  hydrogen.  The  hydrogen 
passes  through  a  delivery  tube  which  is  led  through  the 
other  hole  in  the  stopper.  This  tube  should  just  barely  pass 
through  the  stopper  and  not  be  pushed  down  into  the  liquid, 
as  there  would  then  be  no  escape  for  the  hydrogen.  The 
delivery  tube  leads  the  gas  into  a  trough  or  pan  of  water  in 
which  several  test  tubes  or  wide-mouth  gas  bottles  have 
been  inverted  after  having  been  filled  with  water.  When 
zinc  (preferably  in  a  coarsely  granular  form,  to  expose  a 
large  surface)  has  been  placed  in  the  generating  bottle  and 
covered  with  water  to  seal  the  thistle  tube  and  dilute  sul- 
phuric acid  is  poured  in  through  the  tube,  a  lively  effer- 
vescence begins.  Since  the  bottle  was  filled  with  air  at  the 
outset,  the  gas  which  first  issues  from  the  delivery  tube  will 
be  a  mixture  and  should  be  rejected  because  it  is  explosive 
and  because  it  does  not  have  the  same  properties  as  pure 
hydrogen.  After  a  few  minutes  the  air  will  all  have  been 
displaced,  and  then  one  of  the  bottles  in  the  trough  may  be 
held  over  the  end  of  the  delivery  tube  and  some  hydrogen 
collected. 

113.  Properties.     Hydrogen  burns  with  a  very  hot  but 
almost   colorless   flame.     It   does   not  support   combustion 
when  a  lighted  match  is  thrust  into  it.     It  is  colorless  and 
odorless  when  pure  (although  when  prepared  as  usual  from 
impure   materials,  other  substances  may  be  present  in  it 
which  have  odor  of  their  own). 

114.  Lightness.     Hydrogen   is   the   lightest   gas  known, 
one  liter  of  it  weighing  only  0.09  g.,  that  is,  less  than  nrthnr 
as  much  as  the  same  volume  of  water.     In  virtue  of  its 
lightness,  it  is  used  to  fill  balloons,  for  it  is  less  than  one 
fourteenth  as  heavy  as  the  air  which  it  displaces.     For 
every  liter  of  hydrogen  in  the  balloon,  the  lifting  power  is 


114 


HYDROGEN 


equal  to  the  difference  in  weight  between  a  liter  of  air  and 
a  liter  of  hydrogen,  or,  1.29  —  0.09  =  1.20  grams. 

115.  Diffusibility.  Hydrogen  is  the  most  diffusible  of  all 
gases,  that  is  to  say,  it  passes  most  readily  through  small 
openings  or  through  porous  material.  It  is  well  known 

that  toy  balloons,  which  are 
filled  usually  with  illuminating 
.gas  containing  a  considerable 
amount  of  hydrogen,  soon  lose 
their  lifting  power.  This  is 
very  largely  due  to  the  diffusion 
of  hydrogen  through  the  rubber 
skin  of  the  balloon. 

The  effect  of  diffusion  may 
be  prettily  shown  if  a  large  jar 
of  hydrogen  is  placed  over  an 
unglazed  earthen  cylinder  filled 
with  air  and  mounted  as  shown 
in  Fig.  22.  Hydrogen  diffusing 
into  the  porous  cylinder  faster 
than  air,  which  is  more  slug- 
gish, can  diffuse  out,  a  greater 
pressure  is  created  within  the 


FIG.  22. —  Diffusion  of  Hydrogen 
H,  bell  jar  filled  with  hydrogen. 
A,  unglazed  earthen  cylinder 

sealed  to  vertical  tube  dipping  in    cylinder,   and    this  pressure   is 

conveyed    through    the     tube 

reaching  down  and  dipping  into  the  beaker  of  water.  Thus 
bubbles  of  gas  are  seen  to  rise  in  the  beaker  almost  im- 
mediately after  the  jar  of  hydrogen  is  placed  over  the 
porous  cylinder.  If  after  considerable  hydrogen  has  diffused 
into  the  cylinder  the  jar  is  removed,  the  hydrogen  inside 
the  porous  vessel  will  diffuse  out  into  the  air  faster  than 
the  air  can  get  in  and  there  will  be  less  pressure  inside 


DETONATING  GAS  115 

than  out  so  that  the  water  in  the  beaker  will  be  forced  by 
the  outside  air  pressure  up  into  the  inner  cylinder. 

116.  Liquefaction  and  Solidification.     Besides  being  the 
most  diffusible  of  all  gases,  hydrogen  is,  with  one  exception, 
the  most  difficult  gas  to  liquefy  and  solidify.     It  has  long 
been  known  that  by  great  cold  and  pressure  air  could  be 
condensed  to  a  liquid,  although  it  is  only  comparatively 
recently  that  the  liquefaction  of  air  on  a  commercial  scale 
has  been  possible.     But  it  was  not  until  1898  that  hydrogen 
was  first  liquefied.     Dewar,   an   English   scientist,   accom- 
plished   this.     Under    ordinary    atmospheric    pressure    it 
liquefies  at  about  —  252°  C.,  or  only  21°  above  the  abso- 
lute zero.     If  it  is  highly  compressed,  the  gas  liquefies  more 
easily,  that  is  to  say,  at  a  higher  temperature,  but  above  the 
temperature  of  —  241°  C.  or  32°  absolute,  which  is  known 
as  its  critical  temperature,  it  cannot  be  liquefied  at  all.     It  is 
thus  evident  why,  for  so  long  a  time,  hydrogen  resisted  all 
efforts  to  bring  it  into  the  liquid  form.     Pressure  alone  was 
not  sufficient ;    a  very  low  temperature  was  necessary,  and 
many  attempts  were  made  before  this  was  finally  attained. 
Liquid  hydrogen  freezes  at  —  257°  C.  or  16°  absolute. 

Both  the  liquid  and  the  solid  are  colorless. 

117.  Detonating   Gas.     A   mixture   of   two   volumes   of 
hydrogen  and  one  volume  of  oxygen  (the  ratio  by  volume  in 
which  they  combine  to  form  water)  is  known  as  detonating 
gas   because   it   detonates   or   explodes  'so   violently   when 
ignited  by  a  spark  or  kindled  in  any  other  way.     If  soap 
bubbles  are  blown  with  this  mixture  and  lighted  as  they 
float  away,  they  explode  with  a  deafening  report.     This  ex- 
plosion is  due  to  the  great  heat  liberated  when  the  hydrogen 
combines  with  oxygen.     This  heat  expands  the  resulting 
gaseous  product  (steam)  so  suddenly  and  to  such  an  extent 


116  HYDROGEN 

that  a  violent  blow  is  struck  against  the  surrounding  air, 
thus  producing  the  loud  noise.  Mixtures  of  hydrogen  and 
oxygen  in  other  proportions  than  2  to  1  are  also  more  or 
less  explosive,  as  are  also  mixtures  of  hydrogen  with  air. 
Mixtures  of  illuminating  gas  (which  usually  contain  consider- 
able hydrogen)  with  air  are  similarly  explosive,  hence  the 
need  for  great  caution  in  regard  to  using  an  open  flame  in 
the  vicinity  of  a  gas  leak. 

118.  Oxyhydrogen  Flame.  The  great  heat  of  combus- 
tion of  hydrogen  is  utilized  for  obtaining  very  high  tempera- 
tures by  means  of  the  oxyhydrogen  blowpipe,  a  device  in 
which  hydrogen  and  oxygen  are  led  into  a  small  mixing 
chamber  from  which  they  issue  into  the  flame  (Fig.  23). 


FIG.  23.  —  Oxyhydrogen  Blowpipe. 

The  operation  of  the  blowpipe  is  as  follows  :  first  the  hydro- 
gen cock  is  opened  and  the  gas  is  lighted  and  burns  with  the 
oxygen  of  the  air.  Then  the  oxygen  cock  is  gradually  opened, 
introducing  oxygen  into  the  interior  of  the  flame.  The 
flame  grows  sharp  and  pointed  and  the  inside  blue  cone  re- 
treats more  and  more  with  increasing  supply  of  oxygen  until 
it  is  very  short  and  barely  reaches  beyond  the  orifice.  The 
two  gases  are  then  issuing  in  the  correct  combining  propor- 
tions and  the  most  intense  heat  can  be  obtained  for  they 
are  already  mixed  and  ready  to  combine  as  they  leave  the 
orifice.  It  frequently  happens,  if  the  flame  is  not  skillfully 
manipulated,  that  it  strikes  back  into  the  mixing  chamber 


OXYHYDROGEN   FLAME  117 

and  produces  a  sharp  report  from  the  explosion  there.  The 
mixing  chamber  is  made  so  small,  however,  that  if  the 
flame  accidentally  strikes  back,  it  will  not  produce  a  trouble- 
some explosion. 

119.  With   the   oxy hydrogen    flame,   a    temperature   of 
about  2500°  C.  can  be  obtained,  which  is  sufficient  to  melt 
such   ordinarily  infusible   substances  as   platinum,   quartz, 
and  aluminium  oxide.     The  limit  to  the  temperature  which 
can  be  obtained  is  set  by  the  point  at  which  water  vapor 
itself  will  decompose,  for  when  water  vapor  is  heated   to 
2500°  C.  a  large  part  of  it  is  decomposed  into  hydrogen  and 
oxygen  gases,  by  which  process  it  absorbs  heat  instead  of 
evolving  it.     In  just  the  same  way,  mercuric  oxide  is  de- 
composed by  heat  into  mercury  and  oxygen,  only  this  occurs 
at  a  much  lower  temperature. 

120.  There  are  many  practical  applications  of  the  oxy- 
hydrogen  flame  in  cases  where  a  very  high  temperature  is 
needed.     One  of  the  most  interesting  of  these  is  in  melting 
aluminium  oxide  in  making  artificial  rubies  and  sapphires. 
These  stones  are  composed  of  aluminium  oxide  with  slight 
admixtures  of  other  oxides.     Aluminium  oxide  can  only  be 
melted   at  temperatures  above  2000°  C.     It  is  a  common 
enough  substance  and  in  its  ordinary  form  is  of  very  small 
value;  but  by  melting  it  in  the  oxyhydrogen  flame,  clear, 
transparent,    finely  colored  stones  can  be  made   that   are 
equal  in  beauty  to  the  natural  gems. 

121.  With  air  hydrogen  gives  a  very  hot  flame,  but  not 
so  hot  as  with  pure  oxygen,  because  four  volumes  of  cold 
nitrogen  are  mixed  with  every  volume  of  oxygen  that  enters 
the  flame,  and  a  considerable  part  of  the  heat  of  combustion 
must  be  used  to  bring  this  nitrogen  up  to  the  temperature 
of  the  flame. 


118  HYDROGEN 

122.  Hydrogen  Burns  in  Chlorine.     Not  only  does  hydro- 
gen combine  with  oxygen  but  also  with  other  elements, 
notably  chlorine,  which  is  to  be  studied  a  little  later.     If  a 
jet  of  burning  hydrogen  is  inserted  into  a  jar  of  chlorine 
gas,  it  continues  to  burn,  but  now  combines  with  the  chlorine 
instead  of  with  oxygen,  forming  hydrogen  chloride  instead 
of  hydrogen  oxide  (water).     By  making  a  mixture  of  one 
volume  of  hydrogen  and  one  volume  of  chlorine,  an  explosive 
mixture   is   obtained   which  resembles   in  its   violence   the 
oxygen-hydrogen  mixture. 

123.  Reducing  Action.     On  account  of  its  great  affinity 
for  oxygen  and  for  chlorine,  hydrogen  can,  in  many  cases, 
take  these  elements  away  from  the  metallic  elements  with 
which  they  are  frequently  combined,  thus  leaving  the  metallic 
element  uncombined.     Thus  if  black  copper  oxide  is  placed 
in  a  glass  tube  through  which  hydrogen  is  being  passed  and 
if  the  tube  is  heated,  the  black  oxide  of  copper  changes, 
and  in  its  place  we  see  red  metallic  copper  while  at  the  same 
time  drops  of  water  condense  in  the  cooler  part  of  the  tube, 
showing  that  the  oxygen  of  the  oxide  united  with  the  hydro- 
gen to  form  water.     This  type  of  chemical  action  in  which 
oxygen,  or  an  element  similar  to  oxygen,  is  taken  away  from 
its    combination    with    a    metal    is    called    reduction.     The 
copper  oxide  is  reduced  to  the  metallic  condition  by  the  heated 
hydrogen. 

124.  Gaseous  Fuels.     The  great  heat  of  combustion  of 
hydrogen  makes  it  valuable  for  fuel  purposes.     It  is,  how- 
ever, too  costly  in  the  pure  state  to  use  for  any  except  very 
special  purposes  where  comparatively  small  quantities  are 
needed. 

Many  kinds  of  artificial  gas  such  as  those  used  for  illu- 
minating purposes  contain  large  amounts  of  hydrogen  either 


SUMMARY  119 

in  the  free  state  or  combined  with  carbon  or  both  free  and 
combined.  These  mixtures  make  splendid  fuel,  as  they  give 
great  heat  without  making  smoke  or  leaving  ashes,  the 
products  of  combustion  being  invisible  gases. 

Whenever  it  can  be  obtained,  illuminating  gas  is  used 
rather  than  pure  hydrogen  for  filling  balloons,  as,  being  in 
most  cases  about  one  half  hydrogen,  it  is  fairly  light  and  is 
much  cheaper. 

SUMMARY 

In  decomposing  water,  energy  is  required  to  overcome  the 
attraction  between  oxygen  and  hydrogen.  This  energy  is 
supplied  by  the  electric  current  or  by  the  greater  chemical 
energy  of  some  metal  such  as  sodium,  which  is  more  active 
than  hydrogen. 

Hydrogen  may  be  more  easily  obtained  from  acids  than  from 
water.  Any  active  metal  such  as  zinc  will  displace  the  hydro- 
gen of  an  acid. 

Properties  of  Hydrogen:  Hydrogen  is  the  lightest  and  most 
diffusible  gas  known.  It  is  with  one  exception  the  most 
difficult  gas  to  liquefy.  Free  hydrogen  has  a  strong  tendency 
to  combine  with  oxygen,  and  the  heat  produced  by  the  com- 
bination gives  the  very  high  temperature  of  the  oxyhydrogen 
flame. 

Hydrogen  is  a  reducing  agent,  since  it  can  withdraw  oxygen  from 
its  combination  with  many  of  the  metals  in  their  oxides. 
Hydrogen  forms  compounds  also  with  other  non-metals  than 
oxygen,  notably  with  chlorine. 

Questions 

1.  What  chemical  property  of  hydrogen  was  responsible  for  its 
name? 

2.  What  has  to  be  overcome  to  separate  the  hydrogen  from  the 
oxygen   in   water?     Mention  two   different   methods   for   accom- 
plishing this. 


120  HYDROGEN 

3.  How  is  it  that  sodium  can  decompose  water  when  there  is 
so  strong  a  force  holding  the  hydrogen  and  the  oxygen  together? 

4.  From  what  source  is  hydrogen  released  more  readily  than 
from  water? 

5.  How  could  you  distinguish  hydrogen  from  other  colorless, 
odorless  gases,  such  as  oxygen,  or  nitrogen? 

6.  What  property  of  hydrogen  accounts  for  the  constant  loss 
of  gas  from  a  hydrogen  balloon? 

7.  Why  is  the  flame  of  hydrogen  burning  in  the  air  less  hot 
than  that  obtained  when  hydrogen  burns  in  oxygen? 

8.  What  practical  use  is  made  of  impure  hydrogen? 

9.  Why  is  illuminating  gas  so  much  used  for  filling  balloons? 

10.  Why  do  not  gas  stoves  create  a  smoke  nuisance? 

11.  Under  what  conditions  does  a  gas  flame  smoke?     Explain 
why. 

12.  How  might  metallic  iron  be  obtained  from  ordinary  iron 
rust? 

13.  Some   clean,   dry  iron   filings  which  weigh  5.1300  g.   are 
heated  in  a  current  of  steam  and  afterward  are  found  to  have  in- 
creased in  weight  to  5.8446  g.    What  weight  of  hydrogen  has  been 
liberated  and  what  volume  would  it  occupy  under  standard  con- 
ditions ? 

14.  Eleven  and  two  tenths  liters  of  hydrogen  measured  under 
standard  conditions  are  passed  through  a  heated  tube  containing 
dry  copper  oxide.     What  weight  of  water  could  be  condensed  be- 
yond the  heated  tube? 


CHAPTER  XII 

THE   ATOMIC   THEORY 

WE  have  learned  that  when  chemical  substances  combine 
to  form  definite  new  substances,  they  do  so  in  perfectly 
definite  proportions.  For  example,  in  a  case  already  dwelt 
upon  at  some  length,  one  part  by  weight  of  hydrogen  com- 
bines with  7.94  parts  by  weight  of  oxygen,  no  more,  no  less, 
to  form  8.94  parts  of  water.  Almost  countless  illustrations 
of  similar  definite  combining  proportions  might  be  cited. 

This  definiteness  of  the  combining  proportions  is  a  very 
important  characteristic  of  chemical  compounds,  and  it  is 
one  that  is  often  used  to  distinguish  compounds  from  phys- 
ical mixtures.  Thus,  as  we  have  seen,  one  of  the  reasons 
why  we  believe  air  to  be  merely  a  mixture  of  oxygen  and 
nitrogen  is  that  it  can  contain  these  two  elements  in  varying 
proportions. 

125.  Law  of  Definite  Proportions.  It  is  repeatedly  found 
in  the  study  of  nature  that  a  certain  striking  regularity 
exists  among  a  great  number  of  otherwise  more  or  less  dif- 
ferent phenomena.  When  this  regularity  is  reduced  to 
words,  and  the  statement  so  made  is  found  always  to  be  true, 
this  statement  becomes  known  as  a  law.  The  law  of  defi- 
nite proportions  would  read  thus :  Whenever  two  or  more 
elements  are  combined  with  each  other  in  a  definite  chemical 
compound,  the  weights  of  the  elements  in  that  compound  are 
to  each  other  always  in  the  same  definite  numerical  ratio. 

121 


122  THE   ATOMIC   THEORY 

126.  Hypothesis  and  Theory.     When  such  a  law  exists, 
it  cannot  be  accidental;    there  must  be  some  underlying 
cause  for  it.     Scientific  men  are  impelled  to  speculate  upon 
the  nature  of  things  beyond  what  can  actually  be  seen  and 
felt,  and  thus,  if  possible,  to  reach  a  better  understanding  of 
the  workings  of  nature.     Any  suggestion  as  to  the  under- 
lying causes  of  natural  phenomena  is  known  as  an  hypothesis. 
When  an  hypothesis  has  been  found  to  fit  a  large  number  of 
cases,  and  thus  to  be  of  very  general  importance,  it  becomes 
known  as  a  theory. 

127.  Atomic  Theory  of  Dalton.     In  an  attempt  to  explain 
the  nature  of  matter,  the  atomic  hypothesis  in  a  crude  form 
was   brought   forward   by   the   very   ancient   philosophers, 
prominent  among  whom  was  Democritus  (about  400  B.C.). 
When,  with  the  increase  in  the  knowledge  of  chemical  facts, 
the  law  of  definite  proportions  became  recognized  (late  in  the 
eighteenth  century),  it  was  found  that  the  atomic  hypothesis 
furnished  for  this  law  a  beautiful  explanation.     The  law  of 
multiple  proportions,  which  was  discovered  a  little  later, 
could  likewise  be  explained  by  the  atomic  hypothesis.     Thus 
this  hypothesis  rose  to  the  dignity  of  a  theory  and  is  known 
as  the  Atomic  Theory.     This  theory  is  closely  associated 
with  the  name  of  Dalton,  and  is  often  spoken  of  as  Dalton 's 
atomic  theory  because  it  was  Dalton  who  was  the  first  to 
clearly  see  the  application  of  the  hypothesis  to  the  laws  of 
definite  and  multiple  proportions. 

As  has  already  been  said,  a  crude  form  of  the  atomic  hy- 
pothesis was  advanced  many  centuries  ago  by  the  old  Greek 
philosophers.  They  observed  that  any  body  of  matter 
could  be  divided  into  pieces,  each  piece  could  be  divided 
into  smaller  pieces,  each  of  these  smaller  pieces  could  be 
divided,  and  so  on  until  the  pieces  were  smaller  than  could 


ANALOGY  OF  ATOMIC  THEORY  123 

be  distinguished  with  the  eye.  They  could  not  see  any  limit 
to  the  divisibility,  and  yet  they  could  not  conceive  of  the 
subdivision  going  on  without  limit.  They  therefore  imagined 
the  existence  of  atoms,  very  small  indivisible  particles  of 
which  all  matter  was  composed.1  Bodies  of  matter  were 
simply  aggregates  of  atoms,  and  the  aggregates  could  be 
divided  until  the  single  atoms  were  reached,  but  no  further. 

When  at  a  much  later  day  it  became  recognized  that  there 
were  a  number  of  entirely  distinct  elements  of  which  the 
earth  was  composed,  the  idea  of  atoms  became  more  specific, 
for  it  was  thought  that  the  atoms  of  any  one  element  must 
all  be  alike  but  that  the  atoms  of  different  elements  must 
be  different . 

The  final  form  was  given  to  the  atomic  theory  by  Dalton, 
who  made  it  plain,  that,  when  elements  unite  to  form  a 
compound,  it  must  really  be  the  atoms  which  unite  in  pairs 
or  in  simple  groups.  They  must  therefore  be  regarded  as 
having  some  sort  of  attraction  for  each  other.  Dalton  es- 
pecially emphasized,  as  a  vital  part  of  his  theory,  that  all 
the  atoms  of  a  given  element  are  exactly  alike  in  weight  as 
well  as  in  all  of  their  other  properties,  but  that  atoms  of 
different  elements  differ  in  weight  and  in  many  of  their  other 
properties. 

128.  Analogy  of  Atomic  Theory.  Red  beans  and  green 
peas  could  be  put  into  the  same  pail  and  shaken  together 
and  a  mixture  thus  obtained  that  would  "have  a  mottled  ap- 
pearance, some  of  the  small  bodies  being  red  and  others  green. 
There  might  be  more  beans  or  there  might  be  more  peas 
without  essentially  changing  the  mottled  appearance  of  the 
mixture.  Now  let  us  suppose  a  case  that  would  be  analogous 
to  a  case  of  chemical  combination. 

1  The  word  atom  comes  from  the  Greek,  and  means  indivisible. 


124  THE   ATOMIC   THEORY 

Suppose  that  each  bean  had  a  latent  power  to  attach  to 
itself  just  one  pea,  —  no  more  and  no  less.  Suppose,  further, 
that  this  power  was  very  strong  and  that  at  a  given  signal 
it  might  be  brought  into  operation.  When  the  signal  was 
given,  the  mixture  of  beans  and  peas  would  be  thrown  into 
violent  agitation,  —  the  individual  particles  would  be  pulled 
to  and  fro  in  the  effort  to  attach  themselves  to  their  partners. 
Finally  the  mass  would  come  to  rest  again,  and  if  at  the  out- 
set there  had  been  exactly  the  same  number  of  peas  as  beans, 
we  should  have  now  only  pairs  composed  of  one  bean  and  one 
pea  each,  and  no  unattached  beans  or  peas  would  be  left. 
If  at  the  start  there  had  been  more  beans  than  peas,  there 
would  now  be  some  single  beans  left  unattached ;  if  at  the 
start  there  had  been  more  peas,  there  would  now  be  single  peas. 

The  foregoing  is  a  fair  analogy  to  what  is  supposed  to  take 
place,  according  to  Dalton's  atomic  theory,  when  two  ele- 
ments, for  example  hydrogen  and  oxygen,  unite.  These  two 
gases  may  be  mixed  in  any  proportion  whatever  so  long  as 
no  signal  is  given  for  them  to  enter  into  chemical  combination. 
Only  a  mixture  is  produced  like  that  of  the  beans  and  peas. 
This  mixture  appears  uniform  because  we  are  incapable 
of  distinguishing  particles  so  small  as  the  atoms,  but  if 
our  vision  were  more  powerful,  there  would  doubtless  be 
some  such  mottled  appearance  as  that  of  the  beans  and  peas. 
Now  when  the  signal  for  the  two  elements  to  combine  is 
given,  for  example  when  an  electric  spark  is  passed  through 
the  mixture,  the  agitation  caused  is  so  violent  that  we  see 
the  effect  of  an  explosion.  After  the  explosion  there  is  an 
entirely  different  substance,  as  we  know;  its  smallest  par- 
ticles cannot  be  atoms,  because  they  contain  both  hydrogen 
and  oxygen.  The  smallest  particles  of  the  compound  are 
called  molecules. 


MULTIPLE   PROPORTIONS  125 

We  learned  in  Chapter  X  that  water  contains  two  parts  of 
hydrogen  because  one  part  can  be  set  free  before  and  more 
easily  than  the  other.  Perhaps,  then,  in  the  molecule  of 
water  there  are  two  atoms  of  hydrogen  to  one  atom  of  oxygen. 
If  we  consider  the  red  bean  to  represent  the  oxygen  atom  and 
the  pea  the  hydrogen  atom,  our  new  molecule  may  be  pic- 
tured as  consisting  of  a  bean  with  two  peas  instead  of  one 
attached  to  it. 

129.  Analogy  Applies  to  Multiple  Proportions.  In  our 
analogy  with  beans  and  peas  let  us  make  a  still  further  sup- 
position. After  every  bean  has  found  one  pea  to  which  to 
attach  itself,  suppose  that  it  still  has  some  attractive  power 
for  a  second  pea,  although  considerably  less  than  for  the  first. 
If  there  were  enough  more  peas  in  the  mixture,  each  bean 
might  acquire  two  peas  and  a  new  kind  of  grouping  would 
be  in  evidence  in  which  one  bean  would  be  attached  to  two 
peas.  The  fact  that  almost  the  exact  counterpart  of  this 
analogy  is  frequently  met  in  chemistry  furnishes  us  our 
strongest  reason  for  believing  in  the  essential  truth  of  the 
atomic  theory. 

The  law  of  multiple  proportions  is  illustrated  by  the  two 
oxides  of  carbon  which  were  discussed  in  Chapter  VI.  The 
Law  of  Multiple  Proportions  may  be  stated  as  follows : 

When  two  elements  are  able  to  form  more  than  one  definite 
compound,  if  such  weights  of  each  of  the  compounds  are  taken 
that  they  contain  the  same  weight  of  the  first  element,  then  the 
weights  of  the  second  element  in  the  different  compounds  are 
to  each  other  in  the  ratio  of  small  whole  numbers. 

To  return  to  our  analogy,  let  us  consider  the  two  different 
kinds  of  groups  of  beans  and  peas.  In  each  group  we  had 
one  bean,  but  in  the  second  group  we  had  two  peas  instead 
of  one  as  in  the  first  group.  The  number  of  peas  in  the  two 


126  THE  ATOMIC  THEORY 

groups  are  in  the  ratio  of  small  whole  numbers,  namely  as 
one  is  to  two.  In  carboa  monoxide  let  us  suppose  that  the 
smallest  particle  of  the  compound,  that  is,  the  molecule, 
contains  just  one  atom  of  carbon  and  one  atom  of  oxygen. 
Then  for  carbon  dioxide,  which  we  know  contains  twice  as 
much  oxygen  to  a  given  amount  of  carbon,  we  should  have 
two  atoms  of  oxygen  combined  with  each  atom  of  carbon. 

^-^  ^^^  ^_^  ^^^    x*^s  ^e  can  imagme  the 

f         H  Ik  (        jfl  Ik         j  molecule    as    looking 

^-^^^r^^/  somewhat  as  the  ar- 

FIG.  24.-A  co^eption^of  ;he  structure  rangement   of   beans 

and  peas.  In  the 

diagram  (Fig.  24)  each  white  sphere  is  meant  to  represent  an 
atom  of  oxygen  and  each  black  sphere  an  atom  of  carbon. 

130.  Value  of  Atomic  Theory.  If  no  one  had  ever  con- 
ceived of  the  atomic  theory,  it  seems  as  if  it  would  be  im- 
possible for  us  to  give  any  plausible  explanation  of  the  re- 
markable fact  that  elements  combine  in  definite  and  in 
multiple  proportions.  It  must  never  be  lost  sight  of,  how- 
ever, that  the  atomic  theory  is  only  an  attempt  at  an  ex- 
planation. We  cannot  see  the  atoms,  and  we  shall  probably 
never  be  able  to  verify  by  direct  observation  the  correctness 
of  the  atomic  theory.  The  value  of  the  atomic  theory  is 
that  it  does  explain  satisfactorily  the  facts  regarding  chemical 
combination,  and  thus  explaining,  it  serves  as  a  framework 
for  building  the  structure  of  our  knowledge  of  chemical 
facts. 

It  may  be  stated  that  there  is  a  great  deal  more  evidence 
in  support  of  the  atomic  theory  than  that  given  here.  To 
understand  this  evidence  requires  a  knowledge  of  mathe- 
matics and  of  physical  science;  nevertheless  the  atomic 
theory  has  been  so  strengthened  thereby  that  we  can  as 


MOLECULES  127 

nearly  accept  the  existence  of  atoms  for  an  absolute  certainty 
as  anything  which  it  is  impossible  to  see  by  direct  observation. 

131.  Possible    Complexity   of  Atoms.     It   was   formerly 
supposed   that  the   atoms   were  very  small,   hard,   elastic 
spheres    which    were    absolutely    indivisible.     Recent    dis- 
coveries have  tended  to  show  that  the  atoms  are  really  very 
complex,  but  if  this  is  true,  it  nevertheless  remains  a  fact 
that  we  are  unable  by  any  agency  under  human  control  to 
break  apart  the  atoms  into  smaller  particles  or  to  build  up 
atoms  from  smaller  particles.     So  far  as  the  chemist  is  con- 
cerned the  idea  of  atoms  as  indivisible  particles  may  be  taken 
without  reservation. 

132.  Molecules.     It  is  practically  a  certainty  that  matter 
consists  of  atoms  and  that  there  are  as  many  different  kinds 
as  there  are  elements  (i.e.  about  eighty),  and  that  the  atoms 
of  any  given  element  are  all  exactly  alike  in  weight  and  in 
all  of  their  properties.     When  elements  combine  to  form 
compounds,  it  is  then  really  the  atoms  which  unite  to  form 
atom  groups  and  these  groups  are  the  smallest  particles, 
that  is,  the  molecules  of  the  compounds. 

So  far  we  have  not  shown  any  way  in  which  to  tell  how 
many  of  the  atoms  of  each  element  enter  into  the  formation 
of  the  molecule.  We  have,  however,  spoken  of  the  wonder- 
ful simplicity  shown  in  the  combining  volumes  of  gases,  and 
in  a  later  chapter  we  shall  show  that  much  light  is  thrown 
upon  this  problem  by  a  study  of  these  combining  volumes. 

SUMMARY 

Law  of  Definite  Proportions.  When  two  or  more  elements  are 
combined  with  each  other  in  a  definite  chemical  compound, 
the  weights  of  the  elements  in  that  compound  are  to  each 
other,  always,  in  the  same  definite  numerical  ratio. 

B.  AND  W.  CHEM. 9 


128  THE  ATOMIC  THEORY 

Law  of  Multiple  Proportions.  When  two  elements  are  able  to  form 
more  than  one  definite  compound,  if  such  weights  of  each  of 
the  compounds  are  taken  that  they  contain  exactly  the  same 
amount  by  weight  of  the  first  element,  then  the  weights  of 
the  second  element  in  the  different  compounds  are  to  each 
other  in  the  ratio  of  simple  whole  numbers. 

A  natural  law  is  a  statement  of  a  general  truth  which  is  capable  of 
being  proved  by  direct  observation. 

An  hypothesis  is  a  suggestion  advanced  to  explain  any  truth  of  nature. 

A  theory  is  an  hypothesis  which  has  been  thoroughly  tested  and 
found  to  explain  a  great  number  of  facts.  A  theory  cannot 
become  a  law  because  it  is  incapable  of  proof  by  direct  ob- 
servation. 

According  to  the  atomic  theory,  matter  is  made  up  of  infinitely 
small  particles  called  atoms.  The  atoms  of  each  of  the  ele- 
ments are  alike,  but  different  from  those  of  the  other  elements. 
It  is  the  atoms  themselves  which  are  concerned  in  the  forma- 
tion of  chemical  compounds,  for  different  atoms  attract  and 
hold  each  other  in  small  definite  groups  which  are  called  mole- 
cules and  which  are  the  smallest  possible  particles  of  the  com- 
pound. Thus  the  atomic  theory  explains  the  law  of  definite 
proportions  and  the  law  of  multiple  proportions.  Although 
the  atomic  theory  can  hardly  be  proved  by  direct  observation, 
it  is  abundantly  proved  by  deduction.  It  is  of  the  greatest 
service  in  chemistry,  for  it  furnishes  a  framework  for  building 
the  structure  of  our  knowledge  of  chemical  fact. 

Questions 

1.  On  what  two  groups  of  facts  was  Dalton's  atomic  theory 
based? 

2.  How  does  an  hypothesis  differ  from  a  theory? 

3.  Was  the  idea  of  an  atomic  structure  of  matter  original  with 
Dalton? 

4.  Use  the  atomic  theory  to  explain  what  happens  when  charcoal 
burns. 

5.  Discuss  water  and  hydrogen  peroxide  from  the  viewpoint  of 
the  atomic  theory. 

6.  Of  what  use  is  the  atomic  theory  ? 


CHAPTER   XIII 

HYDROGEN    CHLORIDE 

A  METHOD  of  preparing  hydrogen  chloride  was  touched 
upon  in  the  chapter  on  hydrogen  when  the  ability  of  hydro- 
gen to  burn  in  chlorine  was  described.  Hydrogen  chloride 
is  to  demand  our  attention  in  the  present  chapter. 

133.  Formation  and  Recognition  of  Hydrogen  Chloride. 
For  its  study  in  the  laboratory,  hydrogen  chloride  can  be 
prepared  more  easily  by  another  method  than  the  one  al- 
ready mentioned.     If  a  pinch  of  sodium  chloride,  that  is 
common  salt,  is  placed  in  a  test  tube  and  covered  with  a  few 
drops  of  concentrated  sulphuric  acid,  and  perhaps  warmed 
a  little,  a  colorless  gas  is  evolved  which  has  a  sharp  and  very 
choking  odor.     On  blowing  the  breath  across  the  mouth  of 
the  tube,  a  white  cloud  is  formed;    and  on  bringing  near 
the  mouth  of  the  tube  a  piece  of  filter  paper  wet  with  am- 
monia water,  a  very  dense  white  smoke  appears.     These 
properties  are  very  characteristic  of  hydrogen  chloride  gas 
and  serve  admirably  as  a  means  of  recognizing  it. 

134.  Hydrogen  Chloride  Generator.  *  When  larger  quan- 
tities of  hydrogen  chloride  are  to  be  prepared  in  the  labora- 
tory, a  somewhat  more  elaborate  apparatus  than  the  simple 
test  tube  must  be  used.     Some  coarse  rock  salt  is  placed  in  the 
bottom  of  a  large  flask,  provided  with  a  two-holed  rubber 
stopper.     Through  one  hole  passes  a  thistle  tube  which 
reaches  to  the  bottom  of  the  flask;   through  the  other  hole 

129 


130 


HYDROGEN   CHLORIDE 


passes  a  short  elbow  tube  for  leading  away  the  gas  (see  Fig. 
25).  To  start  the  evolution  of  gas,  concentrated  sulphuric 
acid  is  poured  through  the  thistle  tube  until  it  stands  high 
enough  in  the  flask  to  seal  the  lower  end  of  the  tube.  .In 
order  to  maintain  a  vigorous  evolution  of  gas,  the  flask  is 

warmed  rather  cau- 
tiously with  a  Bunsen 
flame,  when  it  be- 
comes necessary. 

Hydrogen  chloride 
cannot  be  collected 
over  water  because 
of  its  great  solubility. 
Dry  collecting  bottles 
are  therefore  set  up- 
right and  the  gas 
is  led  downward 
through  the  delivery 
tube  to  the  bottom 
of  the  bottle.  The 
hydrogen  chloride  is 
heavier  than  air,  and 
as  it  rises  and  fills 
the  bottle  it  lifts  the 
air  and  forces  it  out. 
Of  course  the  hydro- 
gen chloride  mixes  quite  rapidly  with  the  air  by  diffusion ; 
still  if  the  gas  is  supplied  rapidly  and  the  top  of  the  bottle 
loosely  covered  with  a  glass  plate,  the  gas  can  be  collected 
nearly  pure. 

135.    Great  Solubility.     An  interesting  experiment  show- 
ing the  great  solubility  of  hydrogen  chloride  is  to  close  the 


FIG.  25.  —  Hydrogen  Chloride  Generator. 


GREAT  SOLUBILITY 


131 


neck  of  a  large  bottle  of  the  dry  gas  with  a  one-holed  rubber 
stopper  through  which  passes  a  short  straight  glass  tube 
(Fig.  26).  Now,  on  inverting  the  bottle  and  thrusting  its 
neck  into  water,  a  fountain  soon  begins  to  play  from  the 
narrow  end  of  the  glass  tube  and  this  continues  until  the 
bottle  is  practically  full  of  water.  Hydrogen  chloride  is  so 
very  soluble  that  as  soon  as  a  little  water  enters  the  bottle 
a  nearly  complete 
vacuum  is  created. 
The  pressure  of  the 
air  outside  is  now 
no  longer  counter- 
balanced by  the  pres- 
sure of  a  gas  inside 
the  bottle,  and  water 
is  forced  with  some 
violence  through  the 
nozzle  of  the  tube. 

Hydrogen  chloride 
is  one  of  the  most 
soluble  of  gases ;  one 
cubic  centimeter  of 
water  at  ordinary 
temperature  will  dissolve  450  cubic  centimeters  of  the  gas. 
This  amount  of  gas  weighs  0.8  gram,  and  thus  one  gram  of 
water  will  dissolve  0.8  gram  of  hydrogen  chloride  to  form 
1.8  grams  of  solution. 

The  volume  of  this  solution  is  1.5  cubic  centimeters  and 
thus  the  dissolving  of  the  450  cubic  centimeters  of  the  gas 
in  1  cubic  centimeter  of  water  increases  the  volume  of  the 
latter  by  only  one  half  of  a  cubic  centimeter. 

It  was  stated  in  a  preceding  paragraph  that  hydrogen 


FIG.  26.  —  Fountain  Experiment  showing  Great 
Solubility  of  Hydrogen  Chloride. 


132  HYDROGEN   CHLORIDE 

chloride  was  a  colorless  gas,  and  yet  it  was  also  stated  that 
a  white  cloud  is  seen  when  the  breath  is  blown  across  the 
mouth  of  a  test  tube  from  which  the  gas  is  issuing.  The 
white  cloud,  however,  does  not  consist  alone  of  hydrogen 
chloride,  and  furthermore,  it  is  not  a  gas  at  all  but  it  consists 
of  a  vast  number  of  little  liquid  droplets  which  are  condensed 
from  the  moisture  in  the  breath  together  with  the  hydrogen 
chloride.  These  droplets  consist,  then,  of  a  solution  of 
hydrogen  chloride,  and  their  formation  results  from  the  great 
solubility  of  the  gas. 

It  is  still  strictly  true  that  hydrogen  chloride  gas  is  color- 
less and  invisible.  It  only  becomes  visible  as  a  result  of 
condensation  with  moisture,  but  such  condensation  usually 
occurs  when  the  gas  escapes  into  the  air,  for  the  air  always 
contains  some  water  vapor.  Thus  a  little  cloud  is  almost 
always  seen  above  the  mouth  of  the  bottle  of  concentrated 
hydrochloric  acid  when  the  stopper  is  removed. 

136.  Hydrochloric  Acid.  Hydrogen  chloride,  when  dis- 
solved in  water,  is  most  commonly  known  as  muriatic  acid 
or  as  hydrochloric  acid,  for  when  in  solution  it  possesses 
the  properties  of  a  strong  acid.  In  the  form  of  the  dry  gas, 
it  possesses  no  acid  properties.  Muriatic  acid  is  the  com- 
mercial term,  while  hydrochloric  acid  is  the  scientific  name 
for  the  solution. 

It  is  already  known  from  the  chapter  on  hydrogen  that 
acids  contain  hydrogen  which  can  be  driven  out  by  zinc  and 
some  other  metals.  Two  other  very  characteristic  proper- 
ties of  acids  are  their  sour  taste  and  their  ability  to  change 
the  color  of  litmus  from  blue  to  red.  Litmus  is  a  vegetable 
dyestuff  which  is  very  sensitive  to  acids  and  alkalies,  it  being 
turned  red  by  acids  and  blue  by  alkalies.  Usually  strips  of 
paper  are  colored  with  the  dye,  and  a  strip  of  paper  thus 


USES  OF  HYDROCHLORIC  ACID  133 

colored  blue  will  immediately  change  to  red  when  dipped  in 
acid. 

When  hydrochloric  acid  is  treated  with  pieces  of  zinc,  a 
vigorous  evolution  of  hydrogen  takes  place.  Hydrochloric 
acid  turns  blue  litmus  to  red  and  it  tastes  excessively  sour. 
In  fact,  concentrated  hydrochloric  acid  is  very  corrosive  and 
totally  unsafe  to  taste;  one  part  of  this  acid  diluted  with 
one  hundred  parts  of  water  gives  a  solution  which  will  change 
litmus  to  red  and  which  is  sour  although  quite  safe  to  taste. 

137.  Method  of  Manufacture.     From  a  practical  point 
of  view,  the  dry  hydrogen  chloride  is  of  little  importance, 
but  in  solution  as  hydrochloric  acid  it  finds  extended  use. 
The  method  of  its  manufacture  on  a  large  scale  is  not  different 
in  principle  from  that  of  the  laboratory  preparation.     Com- 
mon salt  is  treated  in  large  cast-iron  pans  with  concentrated 
sulphuric  acid.     The  pans  are  heated  by  a  fire  built  under- 
neath.    When  the  reaction  is  one  half  completed,  the  charge 
in  the  pans  is  raked  upon  hearths  of  fire  brick  where  it  is 
heated  much  hotter  and  the  reaction  is  carried  to  an  end. 
The  gas  is  led  off  through  flues  and  brought  in  contact  with 
water  so  that  it  is  entirely  dissolved.     The  saturated  solu- 
tion that  is  thus  obtained  contains  about  40  per  cent  by  weight 
of  hydrogen  chloride  and  is  known  as  concentrated  hydro- 
chloric acid. 

138.  Uses  of  Hydrochloric  Acid.     Considerable  quantities 
of  hydrochloric  acid  are  manufactured  for  use  in  the  indus- 
trial arts.     Most  of  this  is  sold  in  the  somewhat  impure 
condition  known  as  muriatic  acid.     Some,  however,  is  further 
purified  and  sold  as  C.P.  (meaning  chemically  pure)  hydro- 
chloric acid.     The  acid  is  usually  shipped  in  large  glass 
carboys  packed  in  wooden  boxes  with  some  sort  of  packing 
material  to  prevent  breakage.     Such  carboys  are  to  be  seen 


134  HYDROGEN   CHLORIDE 

at  many  manufacturing  plants.  The  acid  is  used  by  tinmen 
and  plumbers  for  cleaning  metal  surfaces  to  be  soldered. 
Hydrochloric  acid  is  also  used  in  preparing  chlorides  of  the 
metals,  in  preparing  chlorine  and  hydrogen  in  the  laboratory, 
in  making  aqua  regia,  and  in  analytical  work  both  in  schools 
and  colleges  and  in  practical  analytical  laboratories.  The 
use  of  this  acid  is  not  nearly  so  extensive  as  that  of  sulphuric 
acid,  which  is  much  cheaper  arid  serves  equally  well  many  of 
the  purposes  for  which  an  acid  is  required.  Likewise  when 
a  compound  containing  chlorine  is  required,  it  is  customary 
to  use  sodium  chloride  on  account  of  its  cheapness,  rather 
than  the  more  expensive  hydrochloric  acid. 

139.  Nature  of  the  Reaction  in  the  Generator.  Concen- 
trated sulphuric  acid  is  composed  of  hydrogen,  sulphur,  and 
oxygen.  It  may  be  called  hydrogen  sulphate,  as  we  shall 
see  later,  just  as  hydrochloric  acid  is  called  hydrogen  chloride. 
Sodium  chloride,  as  the  name  implies,  is  a  compound  of  sodium 
and  chlorine.  When  sulphuric  acid  and  sodium  chloride 
interact,  the  hydrogen  of  the  former  and  the  chlorine  of  the 
latter  combine  to  form  hydrogen  chloride,  which  in  virtue  of 
the  fact  that  it  is  a  gas,  escapes  readily  from  the  mixture  and 
is  thus  obtained  in  the  pure  condition.  The  sodium  of  the 
sodium  chloride  and  the  sulphur  and  oxygen  of  the  sulphuric 
acid  are  still  to  be  accounted  for.  They  combine  to  form 
another  new  substance,  sodium  sulphate.  The  residue  left 
in  the  generator  after  the  evolution  of  hydrogen  chloride 
according  to  the  laboratory  process,  is  a  solid  substance. 
As  ordinarily  left,  it  will  doubtless  contain  either  some  unused 
sodium  chloride  or  sulphuric  acid,  but  by  a  process  of  puri- 
fication, pure  sodium  sulphate  can  be  separated  from  it. 
This  substance  is  another  go??  and  has  many  similarities  to 
common  salt  in  its  appearance  and  in  other  of  its  properties. 


COMPOSITION  135 

140.  Composition.  We  already  know  from  the  first- 
mentioned  method  of  formation  in  which  hydrogen  is  burned 
in  chlorine,  that  hydrogen  chloride  is  composed  of  hydrogen 
and  chlorine  and  solely  of  these  two  elements.  The  scien- 
tific men  of  earlier  days  who  helped  to  bring  our  science  of 
chemistry  to  its  present  standing,  did  not  possess  as  much 
knowledge  as  we.  They  discovered  the  substance  we  call 
hydrogen  chloride  by  treating  salt  with  sulphuric  acid,  and 
they  called  it  muriatic  acid,  —  meaning  the  acid  obtained 
from  sea  salt.  It  was  found  that  zinc  acting  on  the  acid 
solution  set  free  hydrogen.  It  was  believed  that  zinc  was 
an  element  and  did  not  contain  hydrogen ;  and  it  was  known 
that  zinc  acting  on  water  alone  did  not  liberate  hydrogen. 
Hence  it  was  Ibelieved  that  the  hydrogen  came  from  the 
muriatic  acid.  What  was  apparently  another  constituent 
of  the  muriatic  acid  was  obtained  by  treating  it  with  a  black 
substance  which  we  know  as  manganese  dioxide.  A  greenish- 
yellow,  ill-smelling  gas  was  obtained  which  was  called 
chlorine  on  account  of  its  color  (Moros,  meaning  green). 
Manganese  dioxide  was  known  to  give  off  oxygen  when 
strongly  heated.  Now  possibly  on  reacting  with  muriatic 
acid,  the  manganese  dioxide  gave  up  some  oxygen  to  com- 
bine with  the  hydrogen  of  the  acid  and  form  water,  thus 
leaving  the  residue  of  the  muriatic  acid  uncombined  and 
free  to  escape  as  the  evil-smelling,  greenish-yellow  gas, 
chlorine. 

The  next  step  in  establishing  the  composition  of  muriatic 
acid  was  to  see  if  it  could  be  made  from  hydrogen  and  chlorine. 
This  was,  of  course,  found  to  be  possible,  and  when,  with  fur- 
ther study,  it  was  decided  that  chlorine  as  well  as  hydrogen 
was  an  element,  the  chemical  nature  of  muriatic  acid  was 
definitely  settled. 


136 


HYDROGEN   CHLORIDE 


141.  Combining  Ratio  by  Volume.  When  concentrated 
hydrochloric  acid  is  electrolyzed,  it  is  found  that  hydrogen  is 
evolved  at  the  negative  electrode  and  chlorine  at  the  positive. 
When  water  was  electrolyzed,  we  found  that  two  volumes  of 
hydrogen  and  one  volume  of  oxygen  were  obtained.  We 


FIG.  27.— Electrolysis  of  Hydrochloric  Acid.  E  :  electrolytic  cell;  SS  :  three- 
way  stopcocks  which  may  be  turned  so  that  the  gases  run  to  waste  until 
solution  in  cell  E  has  become  saturated  with  chlorine ;  when  the  gases  are 
being  evolved  at  a  uniform  rate,  the  stopcocks  are  turned  and  the  gases 
collected  in  the  measuring  tubes  C  and  H.  These  tubes  are  filled  with  a 
saturated  salt  solution  which  dissolves  but  little  chlorine. 

find  in  the  case  of  hydrochloric  acid  that  we  obtain  equal 
volumes  of  hydrogen  and  chlorine.  In  order  to  show  this 
fact  satisfactorily,  the  experiment  must  be  performed  in  a 
specially  designed  apparatus  such  as  that  shown  in  Fig.  27. 
Chlorine  is  somewhat  soluble  in  water  and  slightly  so  in 
hydrochloric  acid,  and  if  the  experiment  is  carried  out  in  the 


COMBINING  RATIO   BY   VOLUME 


137 


ordinary  Hofmann  electrolysis  apparatus  (see  page  104),  so 
much  chlorine  is  dissolved  that  a  true  measure  of  its  volume 
is  not  obtained. 

Although  we  obtain  equal  volumes  of  hydrogen  and 
chlorine  from  the  hydrochloric  acid,  we  cannot  be  absolutely 
sure  that  these  gases  have  come  from  the  hydrogen  chloride 
and  not  from  the  water.  If  we  could  now  show  that  equal 
volumes  of  hydrogen  and  chlorine  combine  with  each  other, 


FIG.  28.  —  Collecting  Electrolytic  Hydrogen  and  Chlorine.  E :  electrolytic  cell ; 
A  :  explosion  tube ;  T :  tower  packed  with  lime  and  glass  wool  to  absorb 
waste  chlorine. 

our  proof  would  be  complete.  To  this  end  we  can  obtain 
a  mixture  of  equal  volumes  of  hydrogen  and  chlorine  and 
then  we  pass  the  mixture  through  a  delivery  tube  into  the 
explosion  tube  shown  in  Fig.  28.  When  all  the  air  is 
driven  out  and  the  tube  is  filled  with  the  mixture  at  atmos- 
pheric pressure,  the  stopcocks  at  both  ends  are  closed.  The 
explosion  of  tKis  mixture  can  be  brought  about  even  more 
easily  than  that  of  the  oxygen-hydrogen  mixture.  Exposure 
to  direct  sunlight  or  to  a  magnesium  flashlight  such  as  is 
used  in  photography  is  all  that  is  necessary.  Of  course  the 


138  HYDROGEN  CHLORIDE 

explosion  tube  should  be  strong,  and  furthermore  a  shield 
should  always  be  kept  between  the  tube  and  the  observer. 

If,  as  we  suspect,  the  gases  combine  in  the  ratio  of  one  to 
one  by  volume,  there  should  now  be  no  gas  in  the  tube  except 
hydrogen  chloride.  To  find  the  volume  of  the  product, 
one  end  of  the  tube  may  be  placed  under  concentrated  sul- 
phuric acid  (in  which  hydrogen  chloride  is  not  soluble)  and 
the  stopcock  opened.  We  find  that  the  liquid  does  not  rise 
in  the  tube  nor  do  bubbles  escape  through  the  acid.  Hence 
the  volume  of  the  hydrogen  chloride  at  atmospheric  pressure 
is  just  equal  to  the  sum  of  the  volumes  of  the  hydrogen  and 
chlorine.  To  finally  prove  that  the  gaseous  product  is  hy- 
drogen chloride,  we  dip  the  end  of  the  tube  into  water  and 
find  that  the  water  rises  until  the  entire  tube  is  filled. 

These  experiments  prove  that  one  volume  of  hydrogen  com- 
bines with  one  volume  of  chlorine  to  form  two  volumes  of  hydro- 
gen chloride  gas. 

It  was  found  (p.  103)  that  hydrogen  is  combined  in  two 
different  portions  in  water,  for  one  portion  can  be  displaced 
by  sodium ;  and  from  sodium  hydroxide,  the  new  substance 
formed,  still  another  portion  of  hydrogen  can  be  obtained 
by  treating  this  product  with  zinc. 

In  hydrogen  chloride,  on  the  other  hand,  hydrogen  is  com- 
bined all  in  the  same  portion,  so  far  as  we  can  tell,  for  if 
sodium  reacts  with  hydrogen  chloride,  sodium  chloride  is 
formed  and  hydrogen  is  liberated,  but  the  sodium  chloride 
is  incapable  of  giving  off  any  more  hydrogen. 

142.  Hydrogen  Chloride  and  the  Atomic  Theory.  Viewed 
in  the  light  of  the  atomic  theory,  it  is  obvious  that  every 
molecule  of  hydrogen  chloride  must  contain  hydrogen  and 
chlorine  atoms.  The  simplest  arrangement  that  we  can 
imagine  for  the  molecule  of  the  compound  is  that  it  contains 


SUMMARY  139 

only  two  atoms,  one  of  hydrogen  and  one  of  chlorine.  All 
the  facts  that  we  have  so  far  described  are  in  accord  with 
this  view,  and  unless  further  facts  are  brought  forward  to 
combat  it,  we  shall  adhere  to  this  view  and  write  the  formula 
of  hydrogen  chloride  as  HC1. 

The  combining  ratio  by  weight  of  hydrogen  chloride  is 
1  part  of  hydrogen  to  35.2  parts  of  chlorine.  As  with  hy- 
drogen and  oxygen,  the  ratio  by  weight  is  perfectly  definite, 
although  not  in  simple  whole  numbers  as  the  volume  ratio. 
If  it  is  true  that  hydrogen  and  chlorine  combine  atom  for 
atom,  it  follows  that  the  atom  of  chlorine  weighs  35.2  times 
as  much  as  the  atom  of  hydrogen. 

SUMMARY 

Hydrogen  chloride  is  prepared  by  the  action  of  concentrated  sul- 
phuric acid  on  sodium  chloride. 

Properties  of  Hydrogen  Chloride.  Hydrogen  chloride  is  a  colorless 
gas  with  an  extremely  irritating  odor.  It  forms  a  cloud  with 
moist  air  and  a  dense  white  smoke  with  ammonia.  It  is 
extremely  soluble  in  water  and  is  heavier  than  air. 
In  solution  in  water  it  is  a  strong  acid  and  is  called  muriatic  or 
hydrochloric  acid ;  in  this  solution  it  shows  the  characteristics 
of  acids  in  that  it  reacts  with  zinc,  evolving  hydrogen;  it 
tastes  sour  and  it  colors  blue  litmus  red. 

Composition  of  Hydrogen  Chloride.  Hydrogen  chloride  contains 
only  hydrogen  and  chlorine.  One  volume  of  hydrogen  will 
combine  with  one  volume  of  chlorine  to  yield  two  volumes  of 
hydrogen  chloride. 

By  weight,  hydrogen  chloride  contains  35.2  parts  of  chlorine  to 
one  part  of  hydrogen.     The  weight  relation  is  definite,  al- 
though not  so  simple  as  the  volume  relation. 
It  seems  probable  that  hydrogen  and  chlorine  combine  in  the  ratio 
.  of  atom  for  atom.     The  atom  of  chlorine  weighs,  then,  35.2 
times  as  much  as  the  atom  of  hydrogen  and  the  formula  of 
the  compound  may  be  written  HC1. 


140  HYDROGEN   CHLORIDE 

Questions 

1.  How  is  hydrochloric  acid  made? 

2.  What  elements  does  hydrogen  chloride  contain  and  how  may 
it  be  shown  by  experiment  ? 

3.  What  is  the  volume  relation  of  the  constituents  of  hydrogen 
chloride?     By  what  experiments  can  it  be  shown? 

4.  What  are  the  properties  of  dry  hydrogen  chloride? 

5.  How  do  the  properties  of  hydrochloric  acid  in  solution  differ 
from  those  of  the  dry  gas? 

6.  What  are  the  uses  of  hydrochloric  acid? 

7.  Chlorine  is  allowed  to  enter  a  100  c.c.  evacuated  tube  until 
the  pressure  rises  to  20  cm. ;   then  hydrogen  is  admitted  until  the 
total  pressure  is  76  cm.     After  exploding  the  gas  mixture  what 
will  be  the  total  pressure  of  the  gases  when  they  have  returned  to 
the  initial  temperature  ? 

8.  If  one  end  of  the  tube  from  above  question  is  opened  under 
water  and  the  water  allowed  to  rise  as  far  as  it  will,  what  volume  of 
gas  at  76  cm.  pressure  will  be  left? 

9.  How  does  muriatic  acid  serve  a  useful  purpose  in  soldering 
metals? 

10.   Why  is  less  hydrochloric  than  sulphuric  acid  used  in  the  arts  ? 


CHAPTER   XIV 

AVOGADRO'S    THEORY 

WE  have  seen  in  the  chapter  on  the  atomic  theory  that  all 
matter  is  composed  of  atoms  of  which  there  are  as  many 
different  kinds  as  there  are  different  elements.  Since  com- 
pounds contain  more  than  one  element,  their  smallest  par- 
ticles must  contain  more  than  a  single  atom.  The  smallest 
possible  particles  of  a  compound  are  called  molecules. 

The  atomic  theory  is  almost  a  necessary  consequence  of 
the  law  of  definite  proportions.  This  law  holds  for  all  com- 
binations whether  the  combining  substances  be  solid,  liquid, 
or  gaseous.  But  when  the  substances  are  gaseous,  a  still 
simpler  law  holds.  We  have,  for  example,  seen  that  two 
volumes  of  hydrogen  combine  with  one  volume  of  oxygen 
to  form  water  and  one  volume  of  hydrogen  with  one  volume 
of  chlorine  to  form  hydrogen  chloride. 

143.  Gay  Lussac's   Law   of   Combining  Volumes.     The 
fact  that  gases  combine  according  to  simple  volume  relations 
was  discovered  by  a  French  scientist,  Gay  Lussac,  and  first 
published  by  him  in  1808.     The  Law  of  Combining  Volumes 
which  is  known  as  Gay  Lussac's  law,  may  be   stated   as 
follows : 

Whenever  gaseous  substances  combine,  the  volumes  involved 
are  to  each  other  in  the  ratio  of  small  whole  numbers:  further- 
more, if  the  product  formed  is  also  a  gas,  its  volume  is  to  the 
other  volumes  also  in  the  ratio  of  small  whole  numbers. 

144.  Avogadro's  Hypothesis.     In   order  to   account  for 
these  simple  relations  and  also  to  account  for  the  uniform 

141 


142  AVOGADRO'S  THEORY 

behavior  of  all  gases  under  changes  of  temperature  and  pres- 
sure (Boyle's  and  Charles'  laws)  Avogadro,  an  Italian  phys- 
icist, in  1811  proposed  the  hypothesis  that  equal  volumes  of 
all  gases  under  like  conditions  of  pressure  and  temperature 
contain  the  same  number  of  molecules. 

This  hypothesis,  however,  proves  inadequate  if  one  makes 
the  perfectly  natural  assumption  that  the  molecules  of  the 
elementary  gases  such  as  hydrogen,  oxygen,  and  chlorine 
consist  of  single  atoms.  Avogadro  pointed  out,  as  a  neces- 
sary part  of  his  hypothesis,  that  the  molecules  of  the  com- 
mon elementary  gases,  —  oxygen,  hydrogen,  chlorine,  etc., 
must  contain  two  atoms  each. 

145.  Applications  of  Avogadro's  Hypothesis.  We  know 
that  one  volume  of  hydrogen  combines  with  one  volume  of 
chlorine  to  form  two  volumes  of  hydrogen  chloride.  To 
give  a  clear  picture  of  this,  let  us  use  the  following  diagram, 
in  which  each  of  the  squares  represents  one  volume  of  the  gas  : 


1,000,000 
Molecules 

1,000,000 
Molecules 

— 

2,0 
Mol 

X),( 
eci 

DO 

lies 

1  volume 
hydrogen 

1  volume 
chlorine 

2  volumes 
hydrogen  chloride 

Let  us  assume  that  the  one  volume  of  hydrogen  contains  one 
million  molecules  of  the  substance.1  The  equal  volume  of 
chlorine  would  then  contain,  according  to  Avogadro's  hy- 
pothesis, one  million  molecules  of  chlorine.  The  two  vol- 
umes of  the  gaseous  product  must  then  contain  two  million 

xWe  say  one  million  merely  to  have  some  definite  number  to 
work  with  ;  in  point  of  fact  the  real  number  of  molecules  in  a  liter 
of  hydrogen  under  standard  conditions  has  been  estimated  to  be 
38,000,000,000,000,000,000,000. 


CHEMICAL  SYMBOLS 


143 


molecules  of  hydrogen  chloride.  Now  each  molecule  of 
hydrogen  chloride  must  contain  at  least  one  atom  of  hydro- 
gen, therefore  there  must  have  been  at  least  two  million 
atoms  of  hydrogen  in  the  original  one  million  molecules. 
Similarly  there  must  have  been  two  million  atoms  of  chlorine 
in  the  original  one  million  molecules  of  chlorine.  Therefore 
the  molecules  of  these  elementary  gases  contain  at  least  two 
atoms  each,  as  Avogadro  pointed  out  as  a  necessary  conclu- 
sion from  his  hypothesis. 

146.  Chemical  Symbols.  It  is  customary  in  chemical 
notation  to  use  symbols,  and  the  elements  are  represented 
by  single  letters,  or  at  most  by  two  letters  each,  —  usually 
the  first  letter  of  the  name  of  the  element  is  used.  Thus  H 
stands  for  hydrogen  and  Cl  for  chlorine.  Furthermore,  the 
letter  H  may  be  understood  to  mean  one  atom  of  hydrogen 
and  the  symbol  H2  to  mean  a  group  of  two  atoms,  or  a  mole- 
cule, of  hydrogen.  Using  these  symbols,  we  may  represent 
the  statement  that  one  molecule  of  hydrogen  combines  with 
one  molecule  of  chlorine  to  give  two  molecules  of  hydrogen 
chloride  as  follows : 

H2  +  C12->2HC1. 

Again,  we  know  that  two  volumes  of  hydrogen  combine 
with  one  volume  of  oxygen  to  give  two  volumes  of  water 
vapor.  This  fact  may  also  be  expressed  in  terms  of  the 
hypothesis  as  follows : 

2,000,000  molecules       1,000,000  molecules-      2,000,000  molecules 


+ 

c 

> 

two  volumes 
of  hydrogen 

ne  volume 
of  oxygen 

two  volumes  of 
water  vapor 

or,  reduced  to  simplest  terms, 


B.   AND  W.  CHEM. 


10 


144  AVOGADRO'S  THEORY 

147.  Scope  of  Avogadro's  Theory.  As  we  have  already 
said  in  connection  with  the  atomic  theory,  we  must  not 
allow  ourselves  to  accept  theory  unreservedly  as  we  do 
facts  that  are  proven  by  the  direct  evidence  of  our  senses. 
A  theory  is  merely  a  deduction  which  we  draw  from  facts 
with  the  aid  of  our  imagination.  Avogadro's  theory  belongs 
in  the  same  category  with  the  atomic  theory,  and  the  actual 
molecules,  like  the  actual  atoms,  are  too  small  to  be  seen  or 
directly  counted,  as  we  could  count  peas  in  a  pan  of  peas. 
We  can  thus  scarcely  hope  to  prove  Avogadro's  theory  by  the 
direct  evidence  of  our  senses.  Yet,  like  the  atomic  theory, 
Avogadro's  theory  rests  to-day  on  a  great  deal  more  evidence 
than  that  which  we  have  given.  This  evidence  is  of  so  con- 
vincing a  character  that  the  theory  does  not  fall  far  short  of 
being  a  proven  fact.  Indeed,  many  choose  to  call  it  Avogadro's 
Law,  thus  signifying  their  belief  in  its  absolute  correctness. 

148. l  The  fact  that  all  gases  behave  alike  under  changing 
pressure  and  temperature  (Boyle's  and  Charles'  laws, 
Chapter  VIII)  had  caused  much  speculation  among  scien- 
tific men,  and  it  was  in  fact  largely  through  the  effort  to 
explain  these  laws  that  Avogadro  advanced  his  hypothesis. 
The  necessary  consequence  of  this  hypothesis  that  the  mole- 
cules of  the  elementary  gases  must  contain  two  atoms  ap- 
peared so  improbable  to  the  scientific  men  of  that  time  that 
the  hypothesis  did  not  find  favor  until  many  years  later. 
Although  the  physicists  were  ready  to  admit  that  equal 
volumes  of  gases  under  like  condition  must  contain  the  same 
number  of  particles,  neither  they  nor  the  chemists  were 
clear  as  to  what  the  particles  were  and  the  chemists  were  not 

1  The  remainder  of  this  chapter  may  be  omitted  at  the  discretion 
of  the  teacher,  since  the  understanding  of  later  chapters  does  not 
depend  on  it. 


BOYLE'S  LAW  EXPLAINED  145 

then  ready  to  admit  that  molecules  of  elementary  gases 
could  contain  two  atoms  of  the  same  kind. 

149.  Molecules  of   Gases   must   be   widely   Separated. 
When  we  take  into  consideration  that  water  expands  1700 
times  when  it  is  converted  into  steam,  and  that  liquid  air 
and  other  liquefied  gases  expand  in  similar  degree  when  they 
change  to  gases,  it  becomes  apparent  that  if  matter  really 
does   consist    of   individual    particles,    or   molecules,    there 
must  be  spaces  between  the  molecules  which  are  very  large 
compared  with  the  size  of  the  molecules  themselves. 

150.  Nature  of  Heat.     If  the  particles  of  a  gas  are  so 
widely  separated  (i.e.  when  compared  with  their  size),  what 
can  it  be  that  keeps  them  apart  ?     We  know  that  heat  causes 
the  expansion  of  gases.     Can  it  be,  then,  that  heat  is  a  sort 
of  subtle  fluid  which  flows  in  between  the  particles  of  a  gas 
and  thus  keeps  them  separated?     The  idea  does  not  appear 
absurd,  but  it  has  been  decided  by  physicists  to  be  incorrect. 
Heat  is  believed  to  be  nothing  but  the  energy  of  motion  of 
the  particles  of  a  substance,  moving  in  every  direction,  back 
and  forth,  within  the  limits  set  by  other  particles  with  which 
they  are  continually  coming  into  collision. 

151.  Boyle's  Law  Explained.     Gases  can  exert  enormous 
pressures  when  they  are  confined,  as  is  shown  by  steam  in  a 
steam  engine.     It  is  unsafe  to  confine  liquid  air  in  closed 
vessels,  because,  on  changing  to  a  gas,  jt  develops  such  an 
enormous  pressure  as  to  burst  any  ordinary  steel  cylinder. 
The  pressure  is  caused  by  the  impact  of  the  molecules  of  gas 
as  they  repeatedly  strike  and  rebound  from  the  walls  of  the 
confining  vessel.     The  explanation  of  Boyle's  law  can  now  be 
seen :  when  the  volume  of  a  gas  is  decreased,  the  number  of 
molecules  in  each  unit  of  volume  is  correspondingly  increased, 
and  therefore  the  frequency  of  the  impacts  on  each  unit  of 


146  AVOGADRO'S  THEORY 

surface  is  increased.  Since  the  impacts  constitute  the  pres- 
sure, the  pressure  must  increase  as  the  volume  is  decreased. 
152.  Charles'  Law  Explained.  Heat,  as  already  stated, 
is  nothing  more  or  less  than  the  energy  of  motion  of  the  par- 
ticles of  a  substance.  The  intensity  of  heat  is  measured  by 
the  temperature.  Hence  the  intensity  of  the  energy  of  the 
moving  molecules  must  be  measured  by  the  temperature. 
The  explanation  of  Charles'  law  may  now  be  seen.  The 
intensity  of  the  motion  of  gas  molecules  is  proportional  to 
the  absolute  temperature,  and  thus  the  pressure  of  the  gas 
is  proportional  to  the  absolute  temperature.  At  the  absolute 
zero,  the  gas  would  no  longer  have  any  pressure,  because  its 
particles  would  no  longer  be  in  motion.  The  actual  vol- 
ume of  gas  particles  being  not  more  than  about  one  one- 
thousandth  of  the  volume  that  the  gas  ordinarily  occupies, 
it  is  evident  that  the  volume  would  decrease  to  very  nearly 
zero  at  the  absolute  zero  of  temperature. 

SUMMARY 

Gay  Lussac's  Law.  The  fact  that  the  ratio  of  the  volumes  of  re- 
acting gases  to  each  other  as  well  as  the  ratio  of  either  of  these 
volumes  to  the  volume  of  the  gaseous  product  may  always  be 
expressed  by  small  whole  numbers  was  announced  by  Gay 
Lussac  in  1808. 

Avogadro's  Theory.  Avogadro,  an  Italian  physicist,  in  1811 
proposed  the  hypothesis  that  equal  volumes  of  all  gases  under 
like  conditions  contain  equal  numbers  of  molecules.  This  hy- 
pothesis was  not  well  received  at  first,  but  today  it  is  ac- 
cepted as  a  satisfactory  theory  if  not  indeed  as  a  law. 

Applications  of  Avogadro' s  Hypothesis.     As  a  necessary  deduction 
from  Avogadro's  hypothesis,  the  molecules  of  the  common 
elementary  gases  must  contain  at  least  two  atoms  each. 
The  hypothesis  also  furnishes  a  reasonable  explanation  for  the 
Law  of  Boyle  and  the  Law  of  Charles. 

Heat  is  believed  to  consist  of  the  kinetic  energy  of  moving  molecules. 


CHAPTER  XV 

ATOMIC    AND    MOLECULAR   WEIGHTS 

153.  Absolute  Weights  of  the  Atoms.     The  actual  weight 
of  the  atoms  is  so  exceedingly  small  that  to  give  it  in  figures 
conveys  but  little  meaning  to  one's  intelligence.     For  ex- 
ample, it  has  been  estimated  that  one  atom  of  hydrogen 
weighs  0.000,000,000,000,000,000,000,0012  gram.     For  prac- 
tical purposes,  such  a  figure  can  have  very  little  importance ; 
furthermore,  it  cannot  be  estimated  with  any  great  degree  of 
accuracy.     But  the  relative  weights  of  the  atoms  can  be  deter- 
mined with  a  high  degree  of  accuracy,  and  are  of  great  im- 
portance, as  we  shall  show  in  this  chapter. 

154.  Relative  Weights    of    the   Atoms.     When  different 
elements  combine,  the  atomic  theory  tells  us  that  it  is  really 
the  atoms  which  combine,  and  furthermore,  that  they  com- 
bine in  some  very  simple  numerical  proportion.     What  we 
observe  is  that  tangible  amounts  of  the  different  elements 
disappear  in  the  formation  of  the  compound  ;  but  the  weights 
which  combine  must  be  in  the  same  proportion  or  some  simple 
multiple  of  the  same  proportion  as  the  weights  of  the  actual 
atoms. 

Now  we  know,  for  example,  that  35.2  grams  of  chlorine 
combine  with  1.000  gram  of  hydrogen,  and  furthermore,  we 
have  decided  from  the  volume  composition  of  hydrogen 
chloride  in  connection  with  Avogadro's  theory,  that  hydrogen 
and  chlorine  combine  atom  for  atom.  Hence  the  actual 

147 


148  ATOMIC   AND    MOLECULAR  WEIGHTS 

atom  of  chlorine  must  weigh  35.2  times  as  much  as  the 
actual  atom  of  hydrogen. 

Again,  we  know  that  7.94  grams  of  oxygen  combine  with 
1.000  gram  of  hydrogen,  in  the  formation  of  water,  and  we 
are  convinced  that  each  atom  of  oxygen  combines  with  two 
atoms  of  hydrogen.  Hence  the  atom  of  oxygen  weighs  7.94 
times  as  much  as  two  atoms  of  hydrogen,  or  15.88  times  as 
much  as  a  single  hydrogen  atom.  From  a  comparison  of  these 
figures,  it  follows  that  the  weights  of  the  atoms  of  oxygen 
and  chlorine  are  to  each  other  in  the  ratio  of  15.88  to 
35.2. 

155.  Standard  of  Atomic  Weights.  It  is  perfectly  clear 
that  wre  need  some  measure  by  which  to  express  atomic 
weights,  just  as  the  pound  is  a  measure  for  expressing  quan- 
tities of  sugar  and  flour.  We  have  seen  that  to  give  the 
weights  of  the  single  atoms  in  grams  is  inconvenient,  as  well 
as  uncertain.  But  why  not  take  the  weight  of  the  atom  of 
some  element  as  the  standard  measure?  Although  we  do 
not  know  this  accurately  in  terms  of  the  measure  we  use  for 
ordinary  objects,  we  do  know  accurately  its  ratio  to  the 
weight  of  the  other  atoms,  and  this  is  all  that  is  needed. 
The  weight  of  the  lightest  atom,  that  is  of  the  hydrogen  atom, 
is  the  most  logical  unit  to  choose,  because  when  the  weights 
of  other  atoms  are  expressed  in  terms  of  the  weight  of  the 
hydrogen  atom,  the  numbers  are  all  larger  than  one.  Thus 
if  the  atomic  weight  of  hydrogen  is  1.000,  the  atomic  weight 
of  oxygen  becomes  15.88  and  that  of  chlorine  35.2. 

The  hydrogen  standard  for  atomic  weights  was  formerly 
used  very  generally,  but  at  present  a  slight  modification  of 
this  standard  has  been  adopted  by  almost  all  chemists. 
Oxygen  is  now  made  exactly  16.00  instead  of  being  taken  as 
15.88,  and  the  other  atomic  weights  are  made  to  correspond. 


ATOMIC   WEIGHT  DETERMINATIONS  149 

This  brings  the  figure  for  hydrogen  to  1.008  and  that  for 
chlorine  to  35.45. 

The  standard  O  =  16.00  is  used  instead  of  H  =  1.000 
because  nearly  all  of  the  elements  combine  with  oxygen  and 
because  most  of  the  oxides  can  be  weighed  with  great  ac- 
curacy. Many  of  the  elements  do  not  combine  at  all  with 
hydrogen,  and  in  the  case  of  those  that  do  the  hydrogen  com- 
pounds are  difficult  to  weigh.  Thus  it  is  apparent  that 
oxygen  is  a  far  more  practical  standard  for  comparison. 

156.  Atomic  Weight  Determinations.  To  illustrate  the 
general  mode  of  procedure  in  determining  atomic  weights, 
let  us  imagine  that  we  wish  to  determine  the  atomic  weight 
of  copper.  We  weigh  a  definite  piece  of  pure  copper;  let 
us  suppose  that  it  weighs  1.0000  gram.  Since,  as  we  have  just 
seen,  oxygen  is  the  element  most  used  for  comparison,  we 
will  seek  to  find  what  weight  of  oxygen  is  capable  of  combining 
with  the  copper.  To  do  this  we  convert  the  copper  into 
copper  oxide.  We  must  take  the  greatest  precaution  that 
every  bit  of  the  metal  is  changed  and  also  that  no  particle 
of  the  metal  or  its  compound  is  lost  during  the  process. 
Then  we  weigh  the  copper  oxide,  subtract  the  weight  of  the 
copper  which  was  used,  and  thus  find  the  weight  of  oxygen 
which  was  combined  with  the  copper.  The  entries  which  we 
would  make  in  our  notebooks  and  our  calculations  would 
appear  as  follows : 

Weight  of  copper .'....     1.0000 

Weight  of  copper  oxide 1.2517 

Calculations 

Weight  of  oxygen  =  difference       0.2517 

r^      i  •   •          ,•     copper        1.0000        0  r^o 
Combining  ratio  -          -  =  -         -  =  3.973 
oxygen       0.2517 


150  ATOMIC   AND   MOLECULAR  WEIGHTS 

Atomic  weight  (assuming  that  the  oxide  is  CuO)  = 
3.973  X  16.00  =  63.57. 

Thus  if  we  can  assume  that  copper  and  oxygen  are  com- 
bined with  each  other  in  the  proportion  of  atom  for  atom,  we 
have  found  the  atomic  weight  of  copper  to  be  63.57.  The 
most  careful  study  of  the  compound  has  made  it  reasonably 
certain  that  this  oxide  contains  copper  and  oxygen  atoms 
in  equal  numbers  and  63.57  is  therefore  accepted  as  the 
atomic  weight  of  copper. 

157.  Molecular  Weights.     It  is  true  of  molecules  as  well 
as  of  single  atoms  that  the  actual  weight  is  too  small  and  too 
inaccurately  known  to  be  used  for  any  practical  purpose. 
But  since  molecules  are  made  up  of  definite  numbers  of  atoms, 
and  we  know  accurately  the  weight  of  each  of  the  atoms  in 
terms  of  the  weight  of  the  oxygen  atom  taken  as  16.00,  - 
we  can  also  express  the  weights  of  the  molecules  in  terms  of 
the  same  standard.     Thus  the  molecular  weight  of  oxygen, 
which  has  the  formula  O2,  is  2  X  16.00  =  32.00,  and  the 
molecular  weight  of  hydrogen  chloride,  HC1,  is  1.008  +  35.45 
=  36.46. 

158.  Gram-Molecular   Weights;     Moles.      Atomic    and 
molecular  weights  are  in  themselves  hardly  more  than  ab- 
stract numbers.      Thus  when  we  say  that  32.00  is  the  mo- 
lecular weight  of  oxygen,  we  have  only  expressed  a  number 
which  does  not  indicate  any  tangible  amount  of  the  sub- 
stance.    But  if  we  say  32.00  grams  of  oxygen,  we  have  speci- 
fied a  definite  amount.     Likewise,'  2.016  grams  of  hydrogen 
is    a   definite    amount    of   that    substance.      The   molecular 
weight  in  grams  of  a  substance  is  the  molecular  weight  num- 
ber taken  in  grams,  and  this  quantity  is  so  much  used  by 
chemists  that  the  rather  long  expression  has  been  shortened 
to  the  single  word  mole. 


MOLAL  VOLUME 


151 


The  molecule  is  the  smallest  actual  particle  of  a  substance, 
whereas  the  mole  is  a  quantity  of  the  substance  which  con- 
tains an  inconceivably  great  number  of  molecules.  Yet  a 
mole  of  oxygen  contains  the  same  number  of  molecules  as  a 
mole  of  hydrogen,  or  of  any  other  substance,  however  in- 
conceivably great  that  number  may  be  with  respect  to  our 
ordinary  powers  of  perception.  The  molecule  of  oxygen 
weighs  16  times  (in  round  numbers)  as  much  as  the  molecule 
of  hydrogen;  likewise  two  molecules  of  oxygen  weigh  16 
times  as  much  as  two  molecules  of  hydrogen,  and  so  with 
any  greater  number ;  thus  the  mole  of  oxygen,  which  contains 
something  like  850,000,000,000,000,000,000,000  actual  mole- 
cules, weighs  16  times  as  much  as  the  mole  of  hydrogen  which 
contains  the  same  number  of  actual  molecules  of  hydrogen. 

Molal  Volume.  The  volume  of  a  mole  of  oxygen  is  also 
a  quantity  which  is  of  importance.  One  liter  of  oxygen 
measured  under  standard  conditions  (see  Chapter  VIII)  is 
known  to  weigh  1.4296  grams.  Hence  one  mole,  or  32.00 

32.00  .    r, 

grams,  must  occupy =  22.4  liters. 

1.4.29o 

This  volume  is  known  as  the  gram-molecular  volume,  or 
simply  as  the  molal  volume,  and  it  applies  not  only  to  oxygen 
but  to  all  gases,  because,  according  to  Avogadro's  principle, 
the  same  volume  always  contains  the  same  number  of  mole- 
cules of  a  gas,  whatever  the  gas,  if  the  temperature  and  the 
pressure  are  the  same. 


32.00  grams,  or  one 
mole  of  oxygen. 


71  grams,  or  one 
mole  of  chlorine. 


2.015  grams,  or  one 
mole  of  hydrogen. 


152  ATOMIC   AND   MOLECULAR  WEIGHTS 

Thus  the  weight  of  22.4  liters  of  any  other  gas  is  the  molal 
weight  of  that  gas,  and  when  it  is  desired  to  find  what  is 
the  molecular  weight  of  a  new  gas,  it  is  only  necessary  to 
find  by  measurement  what  is  the  weight  of  22.4  liters,  since 
the  molecular  and  molal  weights  are  numerically  the  same. 

159.  Molecular  Weight  determined  by  Weight  of  Molal 
Volume.      Of  course  the  amount   of  gas  actually  weighed 
need  not  be  exactly  22.4  liters.     For  example,   100  c.c.  of 
chlorine     under    standard    conditions    is    found   to   weigh 
0.3170  gram.       The    mole   of    chlorine  would   then   weigh 

-  -  X  22,400  =  71.0  grams.     Hence  the  molecular  weight 

J-UU 
of  chlorine  is  71.0. 

160.  Use  of  Weight  of  Molal  Volume  in  finding  a  Formula. 
We  have  already  decided  to  write  the  formula  of  water  as 
H2O.     If  we  are  correct  in  this,  the  molecular  weight  of  water 
should  be  18  and  we  should  be  able  to  test  the  correctness  of 
this  figure  by  finding  the  weight  of  the  molal  volume.     Under 
standard  conditions  the  molal  volume  of  any  gas  is  22.4 
liters,  but  water  is  not  a  gas  under  standard  conditions. 
We  may,  however,  calculate  the  molal  volume  for  conditions 
under  which  water  is  a  gas,  and  any  temperature  above  100°  C.  , 
if  the  pressure  is  760  mm.,  would  meet  these  requirements. 
Let  us  choose  the  temperature  273°  C.,  which  is  546°  on  the 
absolute  scale  and  therefore  has  on  this  scale  exactly  twice  the 
value  of  the  standard  temperature  of  273°  absolute.      The 
molal  volume  of  oxygen  under  these  conditions  is  given  by 
the  calculation  : 


volume  (760  mm.  and  273°  C.)  =  22.4  X  —  X         =  44.8  liters. 

i  \y\)      —  /  o 

That  is,  it  is  just  twice  the  molal  volume  under  standard 
conditions.     We  have  now  only  to  find  the  weight  of  water 


MOLECULES  OF  COMMON  ELEMENTARY  GASES      153 

vapor  which  will  occupy  44.8  liters  at  273°  C.  and  760  mm. 
This  weight  proves  to  be  18  grams,  and  thus  we  establish 
the  molecular  weight  of  water  as  18  and  further  confirm 
the  formula  as  H2O. 


44.8  liters 

of  Oxygen  at 

273°C.and  760  ram. 


44.8  liters 

of  Water  Vapor 

at  273°C.and  760  mm. 


32  grams,  or  one  mole  of  oxygen.          18  grams,  or  one  mole  of  water  vapor. 

161.  Reason  for  Believing  that  Molecules  of  Common 
Elementary  Gases  do  not  contain  more  than  Two  Atoms.  It 
was  shown  in  the  chapter  on  Avogadro's  Theory  that  the 
molecules  of  hydrogen  as  well  as  of  chlorine,  oxygen,  and  the 
common  elementary  gases  must  contain  at  least  two  atoms 
each.  We  have  tacitly  assumed  that  they  do  not  contain 
more  than  two  atoms,  and  we  will  now  show  that  this  assump- 
tion is  almost  certainly  correct.  The  proof  depends  on  a 
comparison  of  the  molal  volume  of  the  elementary  gas  with 
that  of  a  large  number  of  its  gaseous  compounds. 

One  molal  volume  of  hydrogen  weighs  2  grams  (in  round 
numbers),  and  hence  the  molecular  weight  is  2.  It  seems 
extremely  probable  that  among  all  the  compounds  of  hydro- 
gen there  are  at  least  some  in  which  only  a  single  atom  enters 
into  the  composition  of  the  molecule.  But  a  very  great 
number  of  gaseous  compounds  of  hydrogen  have  been  studied, 
and  none  has  been  found  in  which  a  volume  of  22.4  liters 
contains  less  than  1  gram  of  hydrogen.  Our  confidence  thus 
becomes  very  great  that  the  atomic  weight  of  hydrogen  is  not 
less  than  one  half  of  the  molecular  weight,  or,  in  other  words, 
that  the  molecule  does  not  contain  more  than  two  atoms. 


154  ATOMIC  AND   MOLECULAR  WEIGHTS 

162.  Quantitative    Significance    of    Chemical    Symbols. 
Although   chemical   symbols   serve   a   valuable   purpose   in 
abbreviating  chemical  names  and  thus  save  much  time  in 
chemical  notation,  this  is  not  by  any  means  their  only  use- 
fulness.    They  serve  also  to  indicate  the  weights  of  the  ele- 
ments in  question. 

Thus  the  formula  of  water,  H2O,  indicates  that  two  atoms 
of  hydrogen  are  combined  with  one  atom  of  oxygen  in  the 
molecule  of  the  substance.  It  indicates,  furthermore,  that 
2X1  parts  by  weight  of  hydrogen  are  united  with  16  parts 
by  weight  of  oxygen.  Since  the  approved  scientific  unit  of 
weight  is  the  gram,  the  parts  by  weight  in  the  formula  may 
be  taken  in  grams.  Then  H2O  signifies  2  grams  of  hydrogen 
and  16  grams  of  oxygen  in  combination,  and  the  whole  formula 
stands  for  2  +  16  =  18  grams,  or  one  mole,  of  the  compound. 

163.  Formulas  contain  Condensed  Information.     We  have 
devoted  a  good  deal  of  attention  in  foregoing  chapters  to 
the  methods  by  which  the  composition  of  water  has  been 
found  and  its  molecular  constitution  deduced.     Practically 
all  of  the  information  so  obtained  is  condensed  in  the  formula 
H2O.     The  formulas  of  other  substances  have  been  obtained 
by  equally  painstaking  and  laborious  processes,  but  we  have 
not  the  time,  nor  is  it  desirable  for  us,  to  go  through  all  the 
steps  for  obtaining  every  formula  that  we  shall  use.     These 
formulas  have  been  derived  through  the  labor  of  competent 
scientific  men,  and  their  labor  would  have  been  in  vain  if  it 
were  still  necessary  for  every  one  who  uses  the  formulas  to 
verify  them  step  by  step.     The  only  way  in  which  scientific 
knowledge  can  advance  is  for  each  student  to  learn  how  to 
grasp  the  knowledge  that  has  already  been  obtained,  so  that 
he  can  use  this  knowledge  as  a  starting  point  in  seeking  to 
discover  new  truth. 


INTERPRETATION  OF  FORMULAS  155 

Formulas  condense  and  systematize  vast  funds  of  chemical 
knowledge,  so  that  when  we  learn  to  readily  interpret 
formulas  we  have  at  our  command  information  which  has 
taken  years  of  effort  to  obtain. 

164.  Interpretation  of  Formulas.  To  understand  the 
meaning  of  formulas,  it  is  only  necessary  for  us  to  remember 
that  each  symbol  signifies  one  atomic  weight  of  the  particular 
element,  and  that  each  subscript  means  that  the  atomic 
weight  of  the  element  which  the  subscript  follows  must  be 
taken  that  number  of  times.  For  example,  the  formula  of 
sulphuric  acid,  H2SO4,  means  that  in  this  substance  the  three 
elements  hydrogen,  sulphur,  and  oxygen  are  present.  Further 
it  means  that  sulphuric  acid  contains  two  atomic  weights  of 
hydrogen,  one  of  sulphur,  and  four  of  oxygen.  Since  the 
atomic  weight  of  hydrogen  is  1,  that  of  sulphur  is  32,  and 
that  of  oxygen  16,  it  follows  that  sulphuric  acid  contains  2 
parts  by  weight  of  hydrogen  to  32  of  sulphur  and  64  of  oxygen, 
making  in  all  98  parts  by  weight.  If  as  is  usually  the  custom, 
these  parts  by  weight  are  to  be  taken  in  grams,  the  whole 
formula  denotes  98  grams  of  sulphuric  acid.  In  working 
with  formulas  after  this  manner,  it  is  often  convenient  to 
write  the  numbers  standing  for  the  weights  of  the  respective 
elements  below  the  symbols,  as  follows: 

H2        S          O4 
(2X1)  + 32 +  (4X16). 


The  atomic  weights  are  understood  in  this  fashion  whenever 
chemical  formulas  are  written,  but  ordinarily  they  are  not 
thus  expressed ;  the  values  for  all  of  the  elements  are  to  be 
found  in  the  table  of  atomic  weights,  a  copy  of  which  is 
printed  inside  the  back  cover  of  this  book.  The  pupil  will 


156  ATOMIC   AND   MOLECULAR   WEIGHTS 

find  occasion  to  use  the  atomic  weights  of  a  few  of  the 
commonest  elements  so  frequently  that  he  will  hardly  fail  to 
memorize  them.  A  table  of  the  most  important  elements 
is  printed  on  page  340. 

165.  Equations.     When   chemical   reactions   take   place, 
the  initial  substances  disappear  but  none  of  the  elements 
are  destroyed ;   they  are  unchanged  in  quantity  and  remain 
as  constituents  of  the  new  substances  which  appear.     If  the 
formulas  of  all  the  substances  involved  are  known,  as  well 
as  the  weight  of  each,  it  is  possible  to  write  the  story  of  the 
reaction  in  an  equation. 

The  formulas  of  each  of  the  initial  substances  are  placed 
on  the  left-hand  side  of  the  equation.  Each  formula  is  taken 
as  many  times  as  there  are  formula  weights.  In  the  same  way 
the  formulas  of  the  substances  formed,  taken  the  appropriate 
number  of  times,  are  placed  on  the  right-hand  side  of  the 
equation.  A  number  placed  before  the  formula  of  a  sub- 
stance means  that  the  whole  formula,  not  merely  a  single 
symbol,  is  to  be  multiplied  by  the  number. 

166.  Example  of  an  Equation.     Let  us,  as  an  illustration, 
see  how  the  reaction  which  occurs  when  common  salt  is 
treated  with  sulphuric  acid  at  a  high  temperature  may  be 
expressed  by  means  of  an  equation.     It  is  known  that  98 
grams  of  sulphuric  acid  react  with  117  grams  of  sodium 
chloride.     Since  the  formula  weights  of  these  two  substances 
are  98  and  58.5,  respectively,  the  formula  of  the  latter  must 
be  taken  twice  if  that  of  the  former  is  taken  once.     The 
products  formed  are  hydrogen  chloride  and  sodium  sulphate, 
in  fact  73  grams  of  the  former  and  142  grams  of  the  latter, 
and  these  amounts  are  equal  to  two  formula  weights  of  the 
hydrogen  chloride  and  to  one  formula  weight  of  the  sodium 
sulphate.     We  may  now  write  the  equation  as  follows : 


CHEMICAL  CALCULATIONS  157 

H2SO2  +  2  NaCl-^2  HC1  +  Na2SO4 

98          2(58.5)         2(36.5)         142 

As  in  mathematical  equations,  the  two  sides  of  chemical 
equations  are  exactly  equal ;  the  laws  of  the  conservation 
of  matter  and  of  the  elements  tell  us  that  none  of  the  elements 
can  have  increased  or  decreased  in  amount  during  the  reac- 
tion. Nevertheless,  it  is  not  very  customary  to  use  the 
equality  sign  in  chemical  equations.  Most  chemical  reactions 
are,  to  some  extent  at  least,  reversible  ;  that  is,  they  may  pro- 
ceed forward  or  backward  according  to  varying  conditions. 
Instead  of  the  equality  sign,  then,  an  arrow  is  more  often 
used  so  as  to  indicate  the  prevalent  direction  of  the  reaction. 

167.  Practical  Bearing  of  Atomic  and  Molecular  Weights. 
The  accurate  knowledge  of  molecular  and  atomic  weights  is 
not  a  matter  merely  of  theoretical  interest,  but  it  is  one  of 
immense  practical  importance ;  for,  with  the  aid  of  it,  chemi- 
cal manufacturers  can  calculate  the  quantities  of  the  dif- 
ferent chemicals  which  they  must  take  in  their  work. 

For  example,  suppose  a  manufacturer  of  carbonated  water, 
who  is  forced  to  prepare  his  own  carbon  dioxide,  has  150  kilos 
of  cracked  marble  (calcium  carbonate)  from  which  to  manu- 
facture carbon  dioxide  gas  for  charging  the  wate^r  (see 
page  60) .  He  must  find  out  how  much  sulphuric  acid  must 
be  taken  to  react  with  the  marble.  He  knows  the  chemical 
formulas  of  all  the  substances ;  he  first  writes  the  equation 
for  the  chemical  reaction ;  he  looks  up  in  the  table  the 
atomic  weights  of  the  elements  and  adds  them  to  give  the 
formula  weights  of  calcium  carbonate  and  sulphuric  acid : 

CaC03         +  H2S04  =  CaSO4+CO2+H2O 

40+12+(3X16)      (2  XI) +32+ (4X16) 

100  98 


158  ATOMIC   AND   MOLECULAR  WEIGHTS 

So  the  quantities  of  calcium  carbonate  and  sulphuric 
acid,  since  one  formula  weight  of  each  is  involved  in  the  re- 
action, are  in  the  ratio  of  100  to  98.  If  a  greater  amount  of 
one  is  taken,  the  amount  of  the  other  must  be  correspondingly 
greater.  Therefore,  to  find  the  amount  of  sulphuric  acid 
required  in  the  above  case,  we  make  the  proportion : 

100  =  98 
150      x 

Solving :    x  =  147.     Thus,  147  kilos  of  sulphuric  acid  are 
required  to  react  with  150  kilos  of  calcium  carbonate. 

The  weight  of  carbon  dioxide  to  be  obtained  in  the  same 
reaction  can  be  calculated  in  a  similar  fashion : 

CO2  =  12  +  (2  X  16)  =  44 

Each  formula  weight  of  calcium  carbonate  gives  one  mole 
of  carbon  dioxide.     Therefore  : 

100^44 
150      x 

Solving :  x  =  66.  Thus,  66  kilos  of  carbon  dioxide  are 
obtained. 

The  manufacturer  might  like  to  know  what  volume  this 
carbon  dioxide  would  occupy  as  a  gas;  he  might  wish  to 
state  in  his  advertisements  how  many  volumes  of  gas  were 
compressed  into  each  volume  of  liquid.  Each  formula 
weight  in  grams  of  calcium  carbonate  gives  one  mole  of  car- 
bon dioxide,  which  under  standard  conditions  occupies  22.4 
liters.  Each  formula  weight  in  kilograms  gives  1000  X  22.4 
=  22,400  liters.  Therefore, 

100  =  22400 
150         x 


OZONE  159 

Solving :  x  =  33,600.  Thus,  33,600  liters  of  carbon  diox- 
ide gas  under  standard  conditions  would  be  obtained. 

If  it  were  of  interest  to  the  manufacturer  to  calculate  the 
weight  of  calcium  sulphate  which  he  could  obtain,  that  could 
be  done  by  a  similar  process,  but  ordinarily  it  is  not  necessary 
to  know  this  quantity,  for  the  calcium  sulphate  is  a  worthless 
product  and  is  thrown  away. 

The  foregoing  illustrations  should  have  made  clear  how  ex- 
tremely important  is  the  knowledge  of  molecular  and  atomic 
weights  for  the  purposes  of  chemical  calculations.  It  would 
be  advisable  for  the  pupil  at  this  point  to  practice  with  a 
number  of  similar  calculations.  He  will  find  several  problems 
at  the  end  of  this  chapter.  He  should  also  make  up  problems 
for  himself  based  on  the  substances  discussed  in  the  earlier 
chapters. 

168.  Ozone.  When  oxygen  is  subjected  to  the  action  of 
electrical  oscillations  obtained  from  a  powerful  induction 
coil  (see  Fig.  29),  it  is  found  to  suffer  a  slight  diminution 

/-  TTn  foit  on  outer  tube ^Oc/ter  Taf>e 

|H,;iii<iiiiiiMHmm»™HJM^^  71/be 

^ v,,,,,,,,,,,,,,:,,,,,:,,,,,,:,,,,,,:,,,,,^^^  OX^S)  Of  -4/A /rtfa 

r/n  fo/7  on  /nner  tube^ 


D/scngrge-  O^on/^ec/  Oxygen  or  Air 

FIG.  29.  —  Ozonizing  Apparatus.  Air  or  Oxygen  is  passed  slowly  through  the 
space  between  two  concentric  glass  tubes.  Electrical  oscillations  (but  no 
actual  passage  of  current)  are  being  caused  in  this  space,  for  the  tinfoil  con- 
ductor inside  the  inner  glass  tube  and  that  outside  the  outer  glass  tube  are 
connected  with  the  terminals  of  an  induction  coil. 

in  volume  and  to  acquire  a  strong  irritating  odor  and  a  num- 
ber of  other  properties  not  possessed  by  ordinary  oxygen. 
For  example,  it  will  quickly  darken  a  lustrous  surface  of  silver 
or  mercury,  due  to  formation  of  a  film  of  oxide.  It  will 

B.   AND  W.  CHEM. 11 


160 


ATOMIC  AND   MOLECULAR  WEIGHTS 


bleach  the  color  from  litmus,  and  it  will  turn  to  a  deep  blue 
some  starch  emulsion  to  which  a  little  potassium  iodide  has 
been  added. 

The  new  substance  which  causes  these  effects  is  called 
ozone,  but  it  is  composed  only  of  oxygen.  If  the  gas  is 
heated  to  300°  C.  or  above,  it  loses  all  its  remarkable  prop- 
erties; the  ozone  has  been  converted  back  into  ordinary 
oxygen.  At  best  only  a  rather  small  proportion  of  the 
oxygen  (about  7  per  cent)  can  be  converted  into  ozone  by 
means  of  electrical  oscillations,  but  if  the  gas  coming  from  the 
ozonizing  tube  is  passed  through  a  tube  cooled  with  liquid 
oxygen,  the  ozone  is  condensed  to  an  intensely  blue  liquid. 

By  allowing  this  liquid  to  boil  (—  119°  C.)  nearly  pure 
ozone  is  obtained  as  a  blue  gas,  but  this  rapidly  changes 
until  only  a  small  per  cent  of  ozone  is  left  if  it  is  allowed  to  rise 
to  the  room  temperature.  Measurements  of  the  weight  per 
liter  of  this  gas  at  a  very  low  temperature  before  it  has  had 
a  chance  to  change  to  any  extent  into  ordinary  oxygen,  show 
that  the  weight  of  the  molal  volume  is  48  grams.  What  is 
the  significance  of  this  in  the  light  of  Avogadro's  theory? 

Simply  that  one  molecule  of  ozone  weighs  —  =1.5  times  as 

o2 

much  as  a  molecule  of  oxygen.  Since  we  believe  the  oxygen 
molecule  to  contain  two  atoms  of  oxygen,  we  must  con- 
clude that  the  ozone  molecule  contains  three  atoms  of 
oxygen  and  write  the  formula  of  ozone  O3. 

The  volume  change  when  ozone  decomposes  to  ordinary 
oxygen  is  as  follows  : 


two  volumes  ozone 


three  volumes  oxygen 


OZONE  161 

Ozone  is  formed  in  small  amounts  in  many  different  ways. 
Electric  sparks  through  the  air  produce  ozone,  and  its  odor 
is  always  noticeable  near  the  brushes  of  electrical  machines. 
When  an  electric  current  is  passed  through  dilute  sulphuric 
acid,  the  oxygen  escaping  from  the  positive  pole  contains 
ozone. 

Ozone  is  a  far  more  vigorous  oxidizer  than  ordinary  oxygen, 
as  shown  by  its  action  on  silver  and  mercury.  The  pure 
ozone,  either  as  the  liquid  or  the  gas,  is  explosive.  These 
facts  all  show  that  it  possesses  more  energy  than  ordinary 
oxygen.  When  it  is  prepared  from  oxygen,  energy  must  be 
supplied  electrically  or  otherwise. 

Ozone  is  also  formed  in  the  presence  of  a  number  of  sub- 
stances, notably  yellow  phosphorus  and  resins,  which  are 
capable  of  undergoing  a  noticeable  slow  oxidation  at  or- 
dinary temperatures.  If  a  stick  of  freshly  scraped  yellow 
phosphorus  is  placed  in  the  bottom  of  a  glass  jar  and  left 
half  covered  with  water,  the  odor  of  ozone  in  the  jar  soon 
becomes  very  strong,  and  a  piece  of  paper  moistened  with 
starch  emulsion  containing  potassium  iodide  is  turned  blue. 
At  the  same  time  the  phosphorus  becomes  covered  with  a 
dull  whitish  coating,  because  of  oxidation.  The  energy 
yielded  by  the  oxidation  of  the  phosphorus  must  be  used  in 
the  change  of  some  of  the  surplus  oxygen  to  ozone. 

The  air  in  pine  and  spruce  forests  is^  regarded  as  being 
especially  invigorating,  and  this  property  is  possibly  due  in 
part  to  traces  of  ozone  formed  in  connection  with  the  slow 
oxidation  of  the  resins  on  the  trees. 

Ozone,  like  hydrogen  peroxide,  is  a  germicidal  agent.  It 
has  been  used  with  marked  success  in  purifying  water  sup- 
plies. A  little  of  it  introduced  into  the  air  of  crowded  school- 
rooms and  public  halls  probably  has  a  good  effect. 


162  ATOMIC   AND   MOLECULAR  WEIGHTS 

SUMMARY 

Atomic  Weights.  The  absolute  weights  of  the  atoms  are  too  small 
to  be  of  any  practical  importance.  The  relative  weights  of 
the  atoms  of  the  different  elements,  on  the  other  hand,  are  of 
the  greatest  importance  and  can  be  determined  with  a  high 
degree  of  accuracy. 

These  atomic  weights  are  based  on  the  standard  of  the  oxygen 
atom,  which  is  taken  as  exactly  16.  By  this  means  the  hy- 
drogen atom,  which  is  the  lightest  atom,  becomes  approxi- 
mately, although  not  exactly,  one. 

The  atomic  weight  of  an  element  is  most  often  found  by  making  a 
very  accurate  quantitative  analysis  of  a  simple  oxygen  com- 
pound of  the  element. 

Molecular  weights  are  based  on  the  same  standard  as  atomic  weights. 
The  gram-molecular  weight  of  a  substance  is  its  molecular 
weight  taken  in  grams ;  for  brevity  this  quantity  is  called  a 
mole. 

Molal  Volume.  The  volume  of  one  mole  of  any  gas  at  standard 
conditions  is  22.4  liters.  This  volume  is  known  as  the  gram- 
molecular  volume  or  the  molal  volume. 

To  find  the  molecular  weight  of  any  substance  that  can  be  vapor- 
ized it  is  only  necessary  to  find  the  weight  of  22.4  liters  under 
standard  conditions,  or  of  the  molal  volume  calculated  to  the 
conditions  of  the  gas  if  the  substance  does  not  vaporize  under 
standard  conditions. 

The  chemical  symbol  of  an  element  signifies  the  atomic  weight  in 
grams  of  the  element. 

The  formula  of  a  substance  signifies  the  formula  weight  in  grams  of 
the  substance.  Formulas  contain  much  information  in  re- 
gard to  the  quantitative  composition  of  the  substances  which 
they  represent. 

Equations  tell  in  abbreviated  form  not  only  the  different  substances 
but  the  weights  of  the  substances  which  take  part  in  chem- 
ical reactions. 

Ozone  is  an  extremely  active  modification  of  oxygen.  Its  formula 
isO3. 


QUESTIONS  163 

Questions 

1.  What  weights  of  each  of  the  following  elements  are  repre- 
sented by  the  following  symbols:  O,  H,  Cl,  Na,  and  Fe? 

2.  What  weights  of  each  of  the  following  substances  are  indi- 
cated by  the  formulas  H20,  HC1,  NH3,  N2,  and  H2S04? 

3.  Forty  grams  of  sodium  hydroxide  are  dissolved  in  water  and 
neutralized   with   hydrochloric   acid,   according   to   the   equation, 
NaOH  +HC1  ->-  NaCl  +H2O.     Neglecting  the  accompanying  water, 
how  many  grams  of  hydrochloric  acid  are  necessary? 

4.  What  volume  of  hydrogen  chloride  gas  under  standard  con- 
ditions is  necessary  to  furnish  the  hydrochloric  acid  needed  in 
Questions? 

5.  What  weight  of  sodium  chloride  would  be  obtained  if  the 
solution  from  Question  3  were  evaporated  to  dryness? 

6.  Calculate  the  weight  of  barium  sulphate  which  would  be 
formed  if  100  grams  of  barium  chloride  were  treated  according  to 
the  reaction,  BaCl2  -f  H2S04-^BaS04  +  2  HC1. 

7.  What  volume  of  carbon  dioxide  under  standard  conditions 
would  be  given  off  if  100  grams  of  sodium  bicarbonate,  NaHC03, 
entered  into  the  reaction,  NaHC03  '+  HC1^C02  +  H20  +  NaCl? 

8.  If  100  c.c.  of  ozonized  oxygen  is  passed  through  a  red-hot  tube 
and  then  cooled  to  the  first  temperature,  it  is  found  to  measure  102  c.c. 
What  per  cent  by  volume  of  the  gas  was  ozone  ? 

9.  If  one  gram  of  liquid  ozone  were  to  evaporate,  what  volume 
would  it  occupy  under  standard  conditions  if  none  of  it  became 
changed  to  ordinary  oxygen  ? 

10.  What  volume  would  it  occupy  after  92  per  cent  was  changed 
to  oxygen? 

11.  How  many  cubic  centimeters  will  100  c.c.  of  air  contract  if  it  is 
subjected  to  electrical  oscillations  in  an  ozonizing  tube  until  3  per 
cent  of  the  oxygen  has  been  converted  into  ozone  (21  per  cent  by 
volume  of  air  is  oxygen)  ? 

12.  Refer  to  statements  as  to  composition  of  hydrogen  peroxide 
on  page  108  and  decide  what  is  the  formula  of  the  substance. 

13.  In  what  respects  are  hydrogen  peroxide  and  ozone  similar  in 
their  properties  ? 


CHAPTER   XVI 

CHLORINE 

IT  was  stated  in  Chapter  XIII  that  chlorine  is  obtained 
from  hydrochloric  acid  by  oxidizing  away  the  hydrogen  of 
the  latter  by  means  of  manganese  dioxide.  Using  now  the 
knowledge  of  chemical  abbreviations  gained  in  the  last 
chapter,  we  can  express  this  reaction  by  means  of  the 
equation : 

4  HC1  +  MnO2-^MnCl2  +  2  H2O  +  C12. 

This  equation  shows  that  water  is  formed  by  the  union  of  the 
hydrogen  and  oxygen,  manganese  chloride  is  formed,  and 
half  of  the  chlorine  from  the  hydrochloric  acid  is  set  free. 

169.  Source  of  Chlorine.  The  source  of  hydrochloric 
acid,  and  in  fact  of  most  of  the  chlorine  of  commerce  as  well 
as  most  of  the  chlorine  compounds,  is  common  salt.  Sodium 
chloride  is  present  in  the  water  of  the  ocean  to  the  extent  of 
about  2.65  per  cent.  It  is  also  found  in  vast  deposits  in  many 
parts  of  the  earth  where  formerly  there  have  existed  salt 
lakes,  or  bays,  the  water  of  which  has  evaporated,  leaving 
solid  salt. 

In  the  course  of  ages  these  deposits  have  frequently  be- 
come covered  with  earth  and  buried  deeply  beneath  the  sur- 
face. Such  salt  beds  may  be  worked  by  mining,  or,  more 
often,  by  sinking  pipes  through  which  water  may  be  intro- 
duced to  the  deposit.  When  the  water  becomes  saturated 

164 


DEACON'S   PROCESS 


165 


with  salt,  it  is  pumped  up  again  and  evaporated  to  obtain 
the  dry  salt.  There  is  a  great  advantage  in  evaporating  this 
concentrated  brine,  which  contains  nearly  40  per  cent  of  salt, 
instead  of  sea  water,  which  contains  less  than  3  per  cent. 


FIG.  30.  —  Vats  for  Evaporating  Brine  in  Salt  Industry. 

Consequently,  sea  water  is  not  much  used  as  a  source  of  salt 
except  in  very  warm  and  sunny  climates. 

170.  Deacon's  Process.  On  the  large  scale,  instead  of 
using  the  oxygen  of  manganese  dioxide  to  oxidize  the  hydro- 
gen of  hydrogen  chloride,  an  ingenious  method  of  using  the 
oxygen  of  the  air  was  devised  by  Deacon  in  1868.  Although 
the  free  oxygen  of  the  air  will  not  oxidize  hydrogen  chloride 
under  ordinary  conditions,  it  will  do  so  when  the  conditions 
are  made  right.  A  careful  study,  by  methods  which  we  shall 
not  attempt  to  discuss  fully,  has  proved  that  oxygen  has  a 
greater  chemical  attraction  for  hydrogen  than  chlorine  has, 
and  thus  it  is  only  to  be  expected  that  oxygen  should  be  able 
to  withdraw  hydrogen  from  its  combination  with  chlorine. 


166  CHLORINE 

Under  ordinary  conditions  this  attraction  is  not  apparent, 
any  more  than  the  attraction  between  oxygen  and  charcoal 
in  the  cold. 

When,  however,  hydrogen  chloride  gas  and  air  are  mixed 
and  drawn  through  heated  tubes  containing  copper  chloride, 
a  reaction  takes  place  and  chlorine  is  liberated  freely : 

O2  +  4HC1->2H2O  +  2C12. 

The  copper  chloride  is  not  changed  by  this  reaction ;  it  acts 
therefore  merely  as  a  catalyzer,  just  as  does  the  manganese 
dioxide  in  the  preparation  of  oxygen  from  potassium  chlorate 
(see  page  40).  In  order  that  the  copper  chloride  shall  work 
efficiently,  a  large  surface  of  it  must  be  exposed  to  the  mixture 
of  gases.  This  is  accomplished  by  saturating  sopie  porous 
material,  such  as  pumice  stone,  with  a  solution  of  the  copper 
salt  and  then  drying  it.  This  method  of  obtaining  chlorine 
from  hydrogen  chloride  is  known  as  Deacon's  process. 

171.  Electrolysis  of  Brine.  At  the  present  day  a  large 
part  of  the  chlorine  of  commerce  is  made  by  the  electrolysis 
of  common  salt  solution.  The  chlorine  is  given  off  at  the 
positive  pole;  at  the  negative  pole  sodium  hydroxide  is 
formed,  while  hydrogen  gas  escapes. 

When  dry  sodium  chloride  melted  by  intense  heat  is 
electrolyzed,  chlorine  and  sodium  are  obtained.  The  attrac- 
tion which  holds  sodium  and  chlorine  in  combination  is  over- 
come by  means  of  the  electric  current.  Since  we  know  that 
sodium  reacts  rapidly  with  water, 

2  Na  +  2  H2O->2  NaOH  +  H2, 

the  formation  of  sodium  hydroxide  at  the  negative  pole  when 
the  water  solution  is  electrolyzed  is  easily  understood. 
Thus  by  the  electrolytic  process  another  valuable  product 


167 

is  obtained  besides  the  chlorine.  Such  a  product  other  than 
the  chief  one  sought  is  known  as  a  by-product.  Many  com- 
mercial processes  of  to-day  yield  large  profits  from  their 
by-products,  and  often  thereby  the  price  of  the  principal 
product  is  materially  lessened. 

172.  Laboratory  Preparation   of    Chlorine.     To   prepare 
chlorine  in  the  laboratory  for  the  purpose  of  studying  its 
properties,  the  older  method  of  oxidizing  hydrochloric  acid 
with   manganese   dioxide   is   suitable.     Manganese   dioxide 
in  small  lumps  is  placed  in  a  generator  flask  like  that  used 
in    preparing   hydrogen    chloride    (see   Fig.    25).      Enough 
hydrochloric  acid  is  poured  in  through  the  thistle  tube  to 
seal  the  latter  and  cover  the  manganese  dioxide.     A  reaction 
begins  at  once,  but  to  yield  a  free  supply  of  chlorine  the  flask 
may  be  cautiously  warmed  with  a  Bunsen  flame. 

Chlorine  gas  cannot  well  be  collected  over  water,  as  it 
dissolves  to  some  extent  in  the  latter.  It  may  be  led  through 
a  delivery  tube  to  the  bottom  of  a  bottle.  When  the  bottle 
is  filled  with  the  greenish-yellow  gas,  a  glass  plate  is  placed 
over  the  top  to  prevent  the  gas  from  diffusing  out. 

All  work  with  chlorine  should  be  done  under  a  hood  with 
a  strong  draft,  so  that  none  of  this  offensive  gas  which  escapes 
from  the  generator  and  from  the  bottles  may  get  out  into  the 
laboratory. 

173.  Physical  Properties.     Chlorine  is  a  gas  of  a  pale 
yellowish-green  color.     It  has  an  offensive  and  suffocating 
odor.     When  inhaled,  even  in  minute  quantities  mixed  with 
air,  it  is  very  irritating  to  the  mucous  membranes ;  when  at 
all  concentrated,  it  is  most  dangerous  to  inhale.     One  volume 
of  water  dissolves  a  little  more  than  two  volumes  of  chlorine ; 
the  solution  is  yellow  and  smells  strongly  of  chlorine.     Chlo- 
rine is  about  two  and  one  half  times  as  heavy  as  air.    The  dry 


168  CHLORINE 

gas  is  much  more  easily  liquefied  than  air;  at  atmospheric 
pressure  it  is  liquid  at  —34°  C. ;  at  ordinary  temperature  it 
is  liquefied  by  higher  pressure,  seven  atmospheres  sufficing 
at  20°  C. 

174.  Chemical  Properties.     Chlorine  does  not  burn  in  the 
air,  neither  is  it  a  supporter  of  the  combustion  of  ordinary 
fuel  materials.     A  lighted  taper  thrust  into  a  bottle  of  chlo- 
rine is  extinguished.    Nevertheless,  chlorine  can,  like  oxygen, 
combine  with  almost  all  of  the  other  elements,  and,  with 
some  of  them,  with  considerable  violence.     For  example, 
if  a  little  powdered  antimony  is  dropped  into  a  jar  of  chlorine, 
a  brilliant  shower  of  sparks  is  instantly  produced. 

175.  Chlorine  and  Metals.     Likewise,  iron,  zinc,  and  cop- 
per, if  very  finely  powdered  so  that  a  large  surface  is  pre- 
sented for  the  chemical  action,  ignite  spontaneously  in  chlo- 
rine.    The  combustion  of  these,  as  well  as  of  other  metals  in 
chlorine,  yields  the  chlorides  of  the  metals.     Thus,  sodium 
chloride  is  formed  when  sodium  is  burned  in  chlorine,  whereby 
our  conclusion  already  made  that  common  salt  is  composed 
of  chlorine  and  sodium  receives  further  confirmation. 

176.  Chlorine   and   Hydrogen.     Chlorine  has  a   strong 
tendency  to   combine  with   hydrogen.     As  already   stated 
(page  118),  a  jet  of  hydrogen  burns  in  a  jar  of  chlorine ;  like- 
wise, a  jet  of  chlorine  will  burn  in  a  jar  of  hydrogen.     An 
explosive  mixture  is  formed  by  chlorine  and  hydrogen,  just 
as  detonating  gas  (see  page  115)  is  formed  by  oxygen  and 
hydrogen,  and  this  mixture  is  naturally  most  explosive  when 
it  contains  the  two  gases  in  the  proportion  in  which  they 
combine,  that  is,  volume  for  volume. 

Such  a  mixture  is  made  to  explode  even  more  easily  than 
the  oxygen-hydrogen  mixture.  To  illustrate  this  let  us  fill 
a  thin  glass  bulb  in  a  shaded  comer  of  the  laboratory  with  a 


CHLORINE  169 

mixture  of  equal  volumes  of  chlorine  and  hydrogen.  Then, 
for  protection,  support  a  pane  of  thick  plate  glass  between 
the  bulb  and  the  observer  and  bring  the  whole  into  the  direct 
rays  of  the  sun.  A  deafening  explosion  occurs  immediately. 
The  light  from  a  magnesium  flashlight  powder  is  also  suffi- 
cient to  explode  the  mixture. 

This  experiment  does  not  necessarily  show  that  chlorine  is 
stronger  than  oxygen  in  its  chemical  affinity  for  hydrogen, 
for  we  have  already  stated  (page  165)  that  the  exact  contrary  is 
the  case ;  the  fact  that  the  attraction  of  the  oxygen  is  really 
greater  is  shown  in  that  the  oxygen-hydrogen  mixture  ex- 
plodes with  more  violence  and  develops  more  heat  thereby. 
Starting  an  explosion  is  something  like  pulling  the  trigger  of 
a  rifle ;  the  pull  necessary  to  release  the  hammer  gives  little 
indication  as  to  the  power  of  the  rifle. 

The  affinity  of  chlorine  for  hydrogen  is  further  shown  in 
the  behavior  of  turpentine,  which  is  a  compound  consisting 
of  only  hydrogen  and  carbon.  If  a  piece  of  filter  paper  is 
wet  with  hot  turpentine  and  then  lowered  into  a  jar  of 
chlorine,  a  flash  followed  by  the  escape  of  dense  black 
smoke  occurs.  The  chlorine  has  combined  with  the  hydro- 
gen of  the  turpentine  and  left  the  carbon  uncombined  to 
appear  as  the  black  smoke : 

C10Hi6  +  8  C12  ->  16  HC1  +  10  C. 

That  hydrogen  chloride  has  also  been  formed  is  indicated 
in  that  a  strip  of  litmus  paper  is  turned  red  when  it  is  mois- 
tened and  then  lowered  into  the  jar. 

177.  Chlorine  and  Carbon.  The  last  experiment  would 
also  indicate  that  chlorine  does  not  unite  directly  with  carbon, 
which  is  much  in  line  with  the  fact  that  ordinary  fuels  do  not 
burn  in  chlorine.  Carbon  does,  to  be  sure,  form  compounds 


170  CHLORINE 

with  chlorine,  of  which  the  most  important  is  carbon  tetrachlo- 
ride,  CC14,  a  liquid  substance  that  is  well  known  and  exten- 
sively used  as  a  solvent  and  as  a  fire  extinguisher.  These 
compounds  are  not  made,  however,  by  direct  synthesis  from 
the  two  elements. 

178.  Chlorine  Water.     For  many  purposes  chlorine  reacts 
more  satisfactorily  in  water  solution  than  in  the  form  of  the 
dry  gas.     Such  a  solution  is  known  as  chlorine  water.     It 
reacts  with  the  precious  metals  which  are  not  affected  by 
ordinary  acids.     Thus  a  piece  of  gold  leaf  is  at  once  dissolved 
by  chlorine  water.     The   chlorine   combines  directly  with 
the  gold,  forming  chloride  of  gold,  and  the  latter  is  soluble  in 
water. 

179.  Aqua  Regia.     Chlorine  may  be  liberated  from  hydro- 
chloric acid  by  oxidation  with  nitric  acid.     A  mixture  of 
three  volumes  of  concentrated  hydrochloric  acid  and  one 
volume  of  concentrated  nitric  acid  is  in  the  correct  proportion, 
and  such  a  mixture  is  known  as  aqua  regia  (literally,  kingly 
water)  because  it  continually  liberates  chlorine,  which  causes 
the  precious  metals  to  form  soluble  chlorides  and  thus  appear 
to  dissolve.     Aqua  regia  is  used  by  jewelers  and  chemists 
when  gold  and  platinum  are  to  be  converted  into  soluble 
compounds. 

180.  Bleaching.     The  most  important  use  of  free  chlorine 
is  in  bleaching  linen  and  cotton  fabrics.     The  natural  fibers 
when  woven  into  cloth  present  a  pale  yellow  or  greenish- 
brown  tinge,  and  this  is  destroyed  by  the  bleaching,  leaving 
the  goods  pure  white. 

Chlorine  water  itself  is  unstable,  as  is  shown  by  the  fact 
that  it  loses  its  yellow  color  in  a  few  hours  or,  at  most, 
days.  It  is,  however,  this  very  instability  which  gives  it 
its  power  to  bleach. 


HYPOCHLOROUS  ACID  171 

When  chlorine  is  dissolved  in  water,  a  small  portion  of  it 
interacts  to  form  hydrochloric  and  hypochlorous  acids : 

H2O  +  C12^±HCH-HC10. 

When  a  small  amount  of  these  products  is  formed,  the  re- 
action ordinarily  stops,  because  whenever  any  considerable 
amounts  of  these  products  are  brought  together,  they  react 
to  form  chlorine  and  water.  Thus  in  the  above  equation 
arrows  are  made  to  point  in  both  directions  because  the  re- 
action may  go  either  way,  according  to  conditions.  What  we 
call  chlorine  water  thus  contains  hydrochloric  acid  and 
hypochlorous  acid  in  addition  to  the  chlorine  and  water. 

181.  Instability  of  Hypochlorous  Acid.  Hypochlorous 
acid  is  a  most  unstable  substance,  and  it  gives  up  its  oxygen 
to  oxidizable  substances  with  extreme  ease.  In  fact,  in  the 
sunlight,  it  breaks  down  rather  rapidly  of  itself : 

2  HC1O  -»  2  HC1  +  O2. 

Thus,  if  we  invert  a  test  tube  of  chlorine  water  in  a  beaker  of 
the  same  solution  and  allow  the  whole  to  stand  in  the  sun- 
light, we  find  in  an  hour  or  so  that  the  yellow  color  of  the 
chlorine  has  gone  and  that  in  the  test  tube  a  few  bubbles  of  a 
gas,  which  proves  to  be  oxygen,  have  collected.  As  fast  as  the 
hypochlorous  acid  is  taken  out  of  the  way  through  the  de- 
composing agency  of  the  sunlight,  the  reaction  of  the  chlorine 
with  the  water  progresses  further,  since  the  tendency  to  op- 
pose this  reaction  is  now  removed.  Thus  all  the  chlorine 
ultimately  disappears. 

The  total  change  effected  by  the  succeeding  reactions  sums 
up  as  follows : 

2  C12  +  2  H20  -*  2  HC1O  +  2  HC1 
2  HC1O  -*  2  HC1  +  Q2 

2  C12  +  2  H2Q  ->  4  HC1  +  O? 


172  CHLORINE 

or  expressed  in  words,  chlorine  has  displaced  the  oxygen 
from  water.  This  is  the  direct  opposite  of  what  takes  place 
in  the  Deacon  process,  but  it  is  to  be  noted  that  this  reaction 
takes  place  in  water  solution,  whereas  the  Deacon  process 
was  carried  out  with  the  dry  gases.  Such  a  difference  of 
conditions  oftentimes  occasions  an  entire  change  in  the 
direction  of  a  reaction. 

182.  Theory  of  Bleaching.     Dry  chlorine  gas  is  incapable 
of  bleaching  dry  fabrics,  as  is  easily  shown  by  suspending  a 
piece  of  colored  calico  in  a  bottle  of  dry  chlorine.     Wet  the 
cloth,  however,  and  lower  it  again  into  the  chlorine  and  it  is 
very  soon  decolorized.     Most  dyestuffs,  both  the  coal-tar 
dyes,  which  are  principally  used  in  coloring  cotton  goods,  and 
the  yellow  and  brown  coloring  matter  of  the  natural  fibers, 
are  very  complex  compounds  containing  carbon  and  hydro- 
gen and  often  nitrogen  and  oxygen.     Very  slight  changes  in 
their  make-up,  such  as  the  addition  of  a  little  oxygen,  often 
completely  change  the  color,  and  most  often  remove  it  al- 
together.    It  is   generally  supposed   that  bleaching  really 
consists  in  adding  oxygen  to  the  dyestuff  molecule.     The 
instability  of  hypochlorous  acid  furnishes  what  is  practically 
atomic  oxygen  ready  to  add  directly  to  the  dyestuff: 

HC1O  +  dyestuff  ->  HC1  +  dyestuff  oxide. 

183.  Bleaching  Powder.     Chlorine  gas  itself  is  most  dis- 
agreeable and  dangerous  to  handle  and  is  difficult  to  trans- 
port and  store.     For  bleaching  purposes,  it  has  therefore 
been  customary  to  absorb  the  chlorine  in  calcium  hydroxide, 
with  which  it  reacts  to  form  what  is  known  as  bleaching 
powder ; 

Ca(OH)2  +  C12  ->  Ca<°  +  H20. 


BLEACHING  POWDER 


173 


This  substance  is  a  dry  powder  ;  it  can  be  kept  indefinitely 
without  decomposition,  and  can  be  handled  and  transported 
without  danger. 

When  it  is  to  be  used  for  bleaching,  it  is  stirred  up  to  a  very 
thin  paste  with  water  and  placed  in  vats,  through  which 
the  goods  are  run  over  mechanical  rollers  (Fig.  31).  The 
bleaching  powder  itself  is  rather  ineffective  when  compared 
with  hypochlorous  acid,  but  it  readily  gives  hypochlorous 
acid  when  treated  with  sulphuric  acid  : 


H2S04  ->  CaS04  +  HC1  +  HC1O. 


It  is  not  advisable,  however,  to  mix  the  bleaching  powder 
and  the  sulphuric  acid  all  at  once.     The  roll  of  cloth  after 


S 


C 


w 


w 


FIG.  31.  —  Bleaching.  A,  unbleached  goods  on  roll;  BB,  vats  of  bleach 
liquid;  SS,  vats  of  dilute  sulphuric  acid;  C,  vat  of  sodium  sulphite; 
WW,  water  for  washing  cloth;  H,  steam  heated  roll;  F,  roll  for  the 
bleached  goods. 

passing  through  the  bleach  vat  is  next  passed  through  a  vat 
containing  very  dilute  sulphuric  acid.  Thus  the  hypochlo- 
rous acid  is  produced  only  within  the  fibers  of  the  cloth  where 
it  will  be  effective. 

In  the  diagram,  the  roll  of  goods  is  represented  as  being 
passed  alternately  through  two  bleach  vats  and  two  sulphuric 


1 74  CHLQRINE 

acid  vats,  then  through  a  bath  of  sodium  sulphite  to  destroy 
any  hypochlorous  acid  which  would  otherwise  rot  the  goods, 
then  through  a  washing  bath  of  water,  and  lastly  over  steam- 
heated  cylinders -to  dry  and  iron  the  cloth. 

Bleaching  powder  is  also  known  as  chloride  of  lime,  and 
under  this  name  it  is  extensively  sold  in  small  cans  for  use  as 
a  deodorizer  and  disinfectant.  It  has  an  intense  and  to  most 
people  disagreeable  odor  which  to  some  extent  resembles 
that  of  chlorine.  The  odor  is  really  that  of  hypochlorous 
acid,  a  little  of  which  exists  even  in  the  unacidified  bleaching 
powder.  Its  deodorizing  power  comes  from  the  same  cause 
as  its  bleaching  power,  namely,  the  readiness  with  which  it 
gives  off  oxygen  to  oxidizable  material.  Noxious  matter  is, 
for  the  most  part,  readily  oxidizable,  and  by  oxidation  is 
converted  into  harmless  substances.  As  a  disinfectant, 
bleaching  powder  kills  disease  germs. 

184.  Liquid  Chlorine.     At  present  much  chlorine  is  con- 
densed to  the  liquid  condition  and  stored  and  shipped  in 
strong  steel  cylinders  instead  of  being  absorbed  by  slaked 
lime.     Such  chlorine  is  very  convenient  for  use  by  skilled 
workmen,    but  the   bleaching   powder    is    safer.     In    some 
towns,  liquid  chlorine,  bought  in  ton  lots,  is  being  used 
to  render  the  municipal  water  supply  safe  for  drinking 
purposes. 

185.  Comparison  of  Oxygen  and  Chlorine.     In  its  gen- 
eral chemical  nature,  chlorine  is  extremely  like  oxygen.     It 
is  a  characteristic  non-metal  and  unites  with  all  the  metals, 
forming   chlorides,   just   as   oxygen   forms   oxides.     In   the 
chlorides  and  oxides,  chlorine  and  oxygen  are  mutually  in- 
terchangeable ;   one  atom  of  oxygen,  however,  is  equivalent 
to  two   atoms   of   chlorine.      Under   most   conditions,   the 
chemical  affinity  of  oxygen  is  somewhat  greater  than  that 


SUMMARY  175 

of  chlorine,  and  oxygen  can  drive  chlorine  out  of  its  com- 
pounds more  easily  than  chlorine  can  displace  oxygen.  In 
its  power  to  combine  with  carbon,  oxygen  stands  consider- 
ably ahead  of  chlorine,  but,  on  the  other  hand,  chlorine  ex- 
cels oxygen  in  its  ability  to  form  compounds  with  the  pre- 
cious metals. 

SUMMARY 

The  chief  source  of  chlorine  and  all  chlorine  compounds  is  common 
salt,  which  is  the  most  abundant  material  containing  chlorine. 

Free  chlorine  is  obtained  by  oxidizing  the  hydrochloric  acid  ob- 
tained from  salt.  The  hydrogen  forms  water  with  oxygen, 
while  the  chlorine  is  left  uncombined. 

But  by  far  the  greatest  part  of  the  chlorine  used  commercially 
is  obtained  by  electrolyzing  common  salt  solution.  By  this 
process,  sodium  hydroxide  is  obtained  as  a  by-product. 

Properties  of  Chlorine.  Chlorine  is  a  greenish-yellow  gas,  heavier 
than  air,  and  somewhat  soluble  in  water.  It  is  irritating 
to  the  breathing  passages  and  is  dangerous  if  inhaled  in 
quantity. 

Chemically,  chlorine  is  a  very  active  non-metallic  element,  resem- 
bling oxygen  in  this  respect.  It  forms  compounds  with  almost 
all  of  the  other  elements.  Towards  hydrogen,  carbon,  and 
the  majority  of  the  elements,  it  is  somewhat  less  active  than 
oxygen;  towards  the  precious  metals,  however,  it  is  more 
active. 

The  principal  use  of  chlorine  is  based  on  its  indirect  bleaching  of 
organic  coloring  matter.  This  property  depends  upon  the 
release  of  oxygen  from  water  by  the  action  of  chlorine,  this 
released  oxygen  being  in  a  condition  to  enter  into  reaction 
far  more  easily  than  atmospheric  oxygen. 

For  practical  bleaching  purposes,  chlorine  itself  is  not  much  used, 
but  rather  bleaching  powder,  which  is  obtained  by  allowing 
chlorine  to  react  with  calcium  hydroxide.  Some  chlorine, 
however,  is  compressed  to  the  liquid  condition  in  strong 
steel  cylinders,  for  transportation  and  use. 

B.  AND  W.  CHEM. 12 


176  CHLORINE 

Questions 

1.  What  compound  occurring  in  nature  is  the  chief  source  of 
chlorine?     Describe  two  totally  different  methods  by  which  free 
chlorine  may  be  obtained  from  it. 

2.  In  obtaining  salt  on  the  large  scale,  why  is  brine  from  salt 
wells  used  rather  than  sea  water? 

3.  What  becomes  of  the  sodium  of  the  salt  in  each  of  the  methods 
by  which  chlorine  is  obtained? 

4.  How  could  you  identify  chlorine  without  making  use  of  the 
properties  of  odor  or  color? 

5.  Give  three  reasons  for  considering  oxygen  a  somewhat  more 
active  non-metallic  element  than  chlorine. 

6.  What  uses  has  chlorine,  in  a  practical  way? 

7.  Give  reasons  for  thinking  that  the  bleaching  of  organic  color- 
ing matter  is  a  process  of  oxidation. 

8.  What  weight  of  manganese  dioxide  must  be  used  to  liberate 
chlorine  from  100  kilograms  of  36.5  per  cent  hydrochloric  acid  ac- 
cording to  the  reaction  4  HC1  +  Mn02  ->  MnCl2  +  2  H20  +  C12? 

9.  What  weight  of  chlorine  will  be  obtained  in  Question  8? 

10.  What  volume  will  the  chlorine  obtained  in  Question  9  oc- 
cupy under  standard  conditions? 

11.  If  we  start  with  the  100  kilograms  of  36.5  per  cent  hydro- 
chloric acid  used  in  Question  8,  and  could  use  sulphuric  acid  and 
manganese  dioxide,  what  is  the  maximum  amount  of  free  chlorine 
that  could  be  obtained?     Write  the   equation   and  compute  the 
amounts  of  sulphuric  acid  and  manganese  dioxide  that  would  be 
required. 

12.  How  would  you  treat  bleaching  powder  in  order  to  obtain 
chlorine  gas?     Give  equations. 


CHAPTER   XVII 

SODIUM 

IN  the  preceding  chapter,  we  have  discussed  chlorine,  a 
non-metal  which  is  one  of  the  constituents  of  common  salt. 
The  other  constituent  of  salt  is  sodium,  a  metallic  element. 
Sodium  is  a  different  sort  of  metal  from  iron,  zinc,  copper, 
and  those  metals  that  are  used  in  everyday  life. 

In  the  first  place,  we  never  see  sodium  in  everyday  life 
because  it  is  such  a  chemically  active  substance  that  it  can- 
not be  kept  in  contact  with  the  atmosphere.  A  little  piece 
of  sodium  left  uncovered  in  a  dish  immediately  loses  its 
bright  metallic  appearance  and  turns  dull,  shortly  a  white 
crust  forms  which  continually  swells  up  with  little  bubbles, 
until  at  last  the  whole  lump  seems  to  liquefy.  This  change 
is  due  to  the  reaction  of  sodium  with  the  water  vapor  of  the 
atmosphere.  We  have  already  seen  (Chapter  X)  that  sodium 
reacts  rapidly  with  cold  water,  displacing  hydrogen  from  the 
latter. 

186.  Preparation  of  Sodium.  When,  melted  sodium 
chloride  is  decomposed  by  electrolysis,  sodium  is  set  free 
at  the  negative  pole,  at  the  opposite  pole  from  that  at  which 
the  chlorine  is  liberated.  But  it  is  a  difficult  matter  to  keep 
sodium  chloride  at  a  high  enough  temperature  to  melt  it 
and  thus  enable  the  electrolysis  to  take  place.  Solid  salt 
is  a  non-conductor  of  electricity  and  thus  cannot  be  elec- 
trolyzed. 

177 


178  SODIUM 

In  the  actual  preparation  of  sodium  by  electrolysis,  sodium 
hydroxide,  NaOH,  commonly  called  caustic  soda,  is  used, 
because  this  melts  at  a  fairly  low  temperature.  When  this 
is  electrolyzed,  oxygen  is  evolved  at  the  positive  pole  and 
sodium  and  hydrogen  are  both  formed  at  the  negative  pole. 
The  hydrogen  escapes,  and  the  sodium  which  is  molten  at 
the  temperature  of  the  bath,  is  drawn  off  and  allowed  to 
solidify  in  appropriate  molds.  The  sticks  of  sodium  so 
prepared  are  preserved  under  kerosene  or  similar  oil  to  pro- 
tect them  from  the  atmosphere. 

187.  Physical    Properties.     Sodium    has    a    silver-white 
color  and  a  metallic  luster.     In  its  possession  of  this  metallic 
luster  and  in  its  ability  to  conduct  electricity,  it  resembles 
all  other  metals.     In  other  respects  it  has  quite  different 
physical  properties  from  those  that  the  everyday  man  as- 
sociates with  metals.     Thus  it  is  very  soft;    it  can  be  cut 
with  a  knife  almost  as  easily  as  cheese ;   it  is  so  light  that  it 
floats  on  water,  and  it  melts  below  the  temperature  at  which 
water  boils. 

188.  Chemical   Properties.     Chemically,    sodium    differs 
from  the  common  metals  in  its  extremely  great  activity. 
As  we  have  seen,  it  rapidly  decomposes  cold  water,  liberating 
hydrogen.    It  burns  intensely  in  air,  forming  sodium  peroxide. 
It  does  not,  however,  ignite  spontaneously,  because  it  be- 
comes covered  with  a  thin  coating  of  oxide  which  protects 
the  metal  underneath.     We  have  already  seen  that  sodium 
combines  with  chlorine  as  well  as  with  oxygen ;  in  fact,  sodium 
has  a  very  strong  tendency  to  unite  with  all  the  non-metallic 
elements. 

Being  itself  far  more  active  than  most  of  the  other  metals, 
sodium  is  capable  of  displacing  the  latter  from  their  com- 
pounds just  as  it  displaces  hydrogen  from  water.  For 


SODIUM   CHLORIDE 


179 


example,  metallic  aluminium  is  formed  when  aluminium 
chloride  and  metallic  sodium  are  heated  together.  The  so- 
dium takes  the  chlorine  away  from  the  aluminium,  whereby 
sodium  chloride  is  formed.  Likewise,  sodium  can  remove 
the  oxygen  from  the  oxide  of  any  less  active  metal. 

COMPOUNDS  OF  SODIUM 

189.  Sodium  Chloride,  or  Common  Salt,  NaCl.  Besides 
being  the  chief  source  of  chlorine,  common  salt  is  also  the 
chief  source  of  sodium.  Salt  also  is  as  useful  a  substance 
as  it  is  abundant.  The  human  blood  contains  0.8  per  cent 
of  sodium  chloride. 

Although  necessary  in  small  amount  in  the  food,  too 
much  salt  is  harmful  and  even  dangerously  poisonous.  Ship- 
wrecked sailors  perish  of  thirst 
because  they  cannot  drink  salt 
water.  Salt  in  any  quantity 
is  also  harmful  to  the  lower 
organisms  that  cause  putre- 
faction and  decay.  Hence  the 
use  of  salt  as  a  preservative  for 
meats,  for  fish,  for  hides,  etc. 

Salt  is  easily  soluble  in  cold 
water,  but,  unlike  most  sub- 
stances, it  is  but  slightly  more 
soluble  in  hot  water.  When 
the  water  of  the  solution  is 
evaporated,  the  salt  crystallizes 
in  cubical  crystals.  These 

FIG.  32. — Hopper-shaped  Salt  Crystals. 

crystals  when  perfect  are  true 

cubes,  but  it  is  seldom  that  the  faces  are  evenly  developed. 

We  frequently  find  the  salt  growing  in  hopper-shaped  crys- 


180  SODIUM 

tals.  Figure  32  shows  the  formation  of  an  idealized  hopper 
crystal. 

190.  Sodium  Bicarbonate,  or  Baking  Soda,  NaHCO3. 
This  substance  has  been  mentioned  in  the  chapter  on  carbon 
dioxide  (pages  59  and  61)  as  the  material  which  is  used  to 
supply  that  gas  in  fire  extinguishers  and  in  baking  powder. 
The  sodium  which  sodium  bicarbonate  contains  is  derived 
from  sodium  chloride  ;  the  carbon  dioxide  may  be  obtained 
by  heating  limestone  (see  page  51,  also  see  next  chapter). 

The  process  of  manufacturing  sodium  bicarbonate  is 
carried  out  as  follows  :  A  saturated  solution  of  sodium  chlo- 
ride containing  about  36  grams  of  sodium  chloride  to  100 
grams  of  water  is  first  prepared.  This  is  saturated  with 
ammonia  and  then  carbon  dioxide  is  passed  into  it.  Sodium 
bicarbonate,  which  is  sparingly  soluble,  thereupon  precipi- 
tates out  in  the  solid  form,  and  after  it  is  drained,  washed, 
and  dried,  it  is  ready  to  be  marketed. 

The  reactions  occurring  in  this  process  are  of  interest. 
First,  ammonia  unites  with  water  to  form  ammonium  hy- 


Ammonium  hydroxide  is  a  base,  and  bases  are  exactly  op- 
posed in  chemical  nature  to  acids. 

Next,  carbon  dioxide  dissolves  and  reacts  with  water  to 
form  the  weak  carbonic  acid.  It  is  characteristic  of  bases 
and  acids  that  they  mutually  neutralize  each  other  and 
thereby  form  a  salt  and  water.  Thus  the  carbonic  acid  is 
neutralized  by  the  ammonium  hydroxide  : 

H(HC03)  +  (NH4)OH->(NH4)(HC03)  +  H2O 

and  the  salt  ammonium  bicarbonate  is  formed. 

Now  this  salt  and  sodium  chloride  are  both  fairly  soluble 


SODIUM  BICARBONATE  181 

in  water.  Any  two  salts  in  solution  are  capable  of  inter- 
changing their  component  parts,  or  radicals.  These  two  may 
interchange  in  this  way,  whereby  ammonium  chloride  and 
sodium  bicarbonate  are  formed.  If  both  of  these  new  salts 
were  as  soluble  as  the  former,  no  change  would  be  seen  by  an 
observer,  but  in  fact  the  sodium  bicarbonate  is  only  sparingly 
soluble  and  hence  it  precipitates  out  as  a  powdery  solid. 


(NH4)(HCO3)  +  NaCl-^Na(HCOa)  |  +  (NH4)C1. 

The  arrow  pointing  downward  signifies  that  the  substance 
precipitates. 

This  process  is  called  the  Solvay  process,  and  it  is  carried 
on  extensively  at  Syracuse,  New  York,  where  an  abundant 
supply  of  concentrated  brine  is  supplied  by  salt  wells.  The 
amount  of  salt  drawn  from  the  wells  amounts  to  several 
hundred  tons,  daily. 

In  order  for  this  process  to  be  possible  from  a  commercial 
standpoint,  it  is  necessary  to  work  up  the  ammonium  chlo- 
ride left  in  the  solution  so  that  all  of  the  ammonia  may  be 
recovered  from  it  and  used  over  again  in  the  process. 

Sodium  bicarbonate  is  easily  decomposed  by  heat  and 
it  gives  off  some  carbon  dioxide  if  thrown  into  boiling 
water.  It  is  completely  decomposed  by  acids  so  that  a 
large  amount  of  carbon  dioxide  is  evolved.  On  this  prop- 
erty depends  the  use  in  baking  powders  and  fire  extinguish- 
ers. When  decomposed  by  the  acid,  it  in  turn  neutralizes 
the  acid.  Thus  sodium  bicarbonate  is  a  good  deal  used  to 
neutralize  acids  where  it  is  essential  not  to  use  any  substance 
which  on  its  own  part  is  corrosive.  For  example,  it  is  used 
to  relieve  burns  on  the  flesh,  as  an  eyewash  when  the  eyes 
are  inflamed  and  develop  an  acidity,  and  internally  as  a 
corrective  for  sour  stomach. 


SODIUM    CARBONATE  183 

191.  Baking  Powder.     The  majority  of  baking  powders 
contain  sodium  bicarbonate,  together  with  some  mildly  acid 
solid  substance  such  as  cream  of  tartar,   or  calcium  acid 
phosphate,   or  alum.     On  wetting  the  mixture  immediate 
reaction  results,  whereby  a  large  volume  of  carbon  dioxide  is 
released.     If  the  powder  has  been  previously  well  blended 
with  the  flour  the  carbon  dioxide  is  scattered  throughout 
the  loaf  and  leavens  it.     Some  other  harmless  product  always 
remains  in  the  dough.     In  the  case  of  a  cream  of  tartar  pow- 
der, potassium  sodium  tartrate  (Rochelle  salt)  is  the  product, 
as  is    shown  by  the  following   equation   for    the  reaction 
between  cream  of  tartar  and  bicarbonate  of  soda. 

NaHC03  +  KH(C4H406)  -^KNa(C4H4O6)  +  CO2  +  H2O. 

Cream  of  tartar  Rochelle  salt 

192.  Sodium  Carbonate,  or  Washing  Soda,  Na2CO3.     A 

large  quantity  of  the  sodium  bicarbonate  manufactured  by 
the  Solvay  process  is  used  as  such,  but  a  far  greater  quantity 
is  converted  into  sodium  carbonate,  Na2COs.  On  heating, 
the  bicarbonate  decomposes  into  the  carbonate  while  water 
vapor  and  carbon  dioxide  are  evolved. 

2  NaHC03  -+  NaaCOs  +  H2O  f  +  CO2  f 

The  arrows  pointing  upward  indicate  that  the  substances 
escape  as  gases.  The  carbon  dioxide  thus  freed  is  returned 
into  the  Solvay  process  for  the  conversion  of  more  sodium 
chloride  into  bicarbonate. 

Sodium  carbonate  is  also  called  washing  soda  because  this 
name  describes  one  of  its  important  household  uses. 

Sodium  carbonate  acts  as  a  mild  alkali,  one  indication  of 
this  being  that  it  colors  litmus  blue  in  distinction  to  acids, 
which  color  it  red.  The  use  of  sodium  carbonate  for  cleans- 
ing purposes  depends  on  its  alkaline  character,  for  alkalies 


184  SODIUM 

possess  the  power  of  loosening  grease  and  dirt  and  thus  allow- 
ing of  their  removal  with  water.  Sodium  carbonate  is  suffi- 
ciently alkaline  for  the  purpose  and  yet  not  so  alkaline  as 
to  injure  seriously  the  flesh  or  fabrics  or  woodwork.  Caustic 
soda,  on  the  other  hand,  is  too  powerful  an  alkali  to  use  in 
cleaning,  for  besides  removing  dirt,  it  attacks  the  flesh 
strongly  and  it  disintegrates  woolen  fabrics  and  wood. 

The  alkaline  character  of  sodium  carbonate  is  further 
exhibited  in  its  power  to  neutralize  acids.  Except  for  very 
delicate  neutralizations,  it  is  used  for  this  purpose  more 
extensively  than  sodium  bicarbonate.  Its  action  is  similar 
to  that  of  the  latter,  in  that  a  neutral  salt,  water,  and  carbon 
dioxide  are  formed. 

NajCOs  +  H2SO4->-  Na2SO4  +  H2O  +  CO2. 

But  it  will  be  noticed  on  comparing  the  reactions  that  only 
one  half  as  much  carbon  dioxide  escapes  for  the  same  amount 
of  acid  neutralized. 

Sodium  carbonate  has  a  multitude  of  industrial  uses  which 
for  the  most  part  depend  on  its  alkalinity  and  its  ability  to 
neutralize  acids.  Enormous  quantities  also  are  used  to 
manufacture  sodium  hydroxide  or  caustic  soda,  in  the  manu- 
facture of  soap,  and  in  the  manufacture  of  glass. 

193.  Soda  Crystals,  Na2CO3-io  H2O.  Much  of  the 
sodium  carbonate  of  commerce  is  put  on  the  market  as  a  dry 
powder  of  the  formula  Na2CO3.  But  another  large  part  is 
sold  as  so-called  soda  crystals  which  consist  in  fact  of  trans- 
parent crystals  which  contain  ten  molecules  of  water  in  addi- 
tion to  the  sodium  carbonate  proper,  as  is  indicated  in  the 
above  formula. 

When  a  solution  of  sodium  carbonate  in  water  is  allowed 
to  crystallize  at  a  low  temperature,  the  solid  that  separates 


SODIUM   HYDROXIDE,  OR  CAUSTIC   SODA  185 

contains  ten  moles  of  water  for  each  formula  weight  of  sodium 
carbonate.  This  is  regarded  as  a  true  chemical  compound, 
for  it  always  contains  sodium  carbonate  and  water  in  exactly 
the  same  proportions.  Furthermore,  the  crystals  are  totally 
different  in  structure  from  anhydrous  sodium  carbonate 
(that  is,  Na2CO3  without  water)  or  from  ice  (that  is,  crystal- 
lized water).  The  chemical  affinity  binding  the  water  and 
the  sodium  carbonate  together  is,  however,  much  weaker  than 
that  which  holds  the  hydrogen  and  oxygen  together  in  water, 
or  the  sodium,  carbon,  and  oxygen  in  the  sodium  carbonate. 
That  the  water  is  bound  very  loosely  to  the  salt  in  the 
crystals  is  shown  by  the  fact  that  it  can  all  be  driven  off  with 
very  gentle  heating,  thus  leaving  the  salt  anhydrous.  In 
fact,  this  decomposition  takes  place  slowly  even  at  ordinary 
temperatures,  and  the  crystals  crumble  to  a  white  powder 
consisting  of  the  more  or  less  dehydrated  substance. 

194.  Crystal  Hydrates  and  Efflorescence.     When  crystals 
of  this  type  decompose  spontaneously  in  this  way,  they  are 
said  to  effloresce  (literally  to  bloom)   because  the  powder 
frequently  resembles  masses  of  snowy  blossoms. 

Water  held  in  combination  with  a  salt  in  crystal  form  is 
known  as  water  of  crystallization.  Such  salts  are  called  crystal 
hydrates.  A  great  many  salts  besides  sodium  carbonate 
possess  the  power  of  thus  taking  on  water  of  crystallization. 

195.  Sodium  Hydroxide,  or  Caustic  Soda,  NaOH.     This 
substance  is  obtained  in  the  solution  when  metallic  sodium 
reacts  with  water  and  displaces  one  half  of  the  Iwdrogen. 
It  is  made  commercially  by  the  electrolysis  of  sodium  chloride 
solution.     Probably  the  largest  part  of  the  sodium  hydroxide 
used  is  made  by  causticizing  sodium  carbonate. 

In  the  last-mentioned  method,  the  source  of  the  sodium  is 
sodium  carbonate  and  that  of  the  hydroxide  radical  is  cal- 


186  SODIUM 

cium  hydroxide  (see  next  chapter).  The  success  of  this 
method  depends  on  the  fact  that  the  unwished-for  con- 
stituents, that  is,  the  carbonate  radical  and  the  calcium,  form 
together  a  very  insoluble  substance,  calcium  carbonate, 
which  precipitates,  and  thus  can  be  separated  from  the  sodium 
hydroxide  remaining  in  solution  : 

Na2C03  +  Ca(OH)2  ->  CaCO3 |  +  2  NaOH. 

To  carry  out  the  process,  a  solution  of  sodium  carbonate  is 
thoroughly  mixed  with  calcium  hydroxide.  The  precipitate 
is  allowed  to  settle  and  the  clear  liquor  containing  the  sodium 
hydroxide  is  drawn  off.  This  liquor  is  boiled  in  iron  pots 
until  all  the  water  is  driven  off,  when  pure  sodium  hydroxide 
is  left  as  a  melted  substance.  This  is  then  poured  off  into 
molds  or  into  large  iron  drums,  in  which  it  solidifies. 

Sodium  hydroxide  is  a  white  solid  substance  which  is 
extremely  soluble  in  water.  It  melts  very  easily  at  a  tem- 
perature below  a  visible  red  heat.  It  has  a  great  attraction 
for  water,  as  is  shown  by  the  experiment  of  leaving  a  small 
lump  of  sodium  hydroxide  exposed  to  the  air.  The  surface 
of  the  lump  almost  immediately  becomes  moist  and  soon  the 
whole  lump  appears  to  liquefy.  The  sodium  hydroxide 
dissolves  in  the  water  which  it  absorbs  from  the  atmos- 
phere. Moist  sodium  hydroxide  also  absorbs  carbon  dioxide 
with  great  avidity  and  thereby  forms  sodium  carbonate. 

2  NaOH  +  CO2->Na2CO3  +  H2O. 

Thus  if  a  lump  of  sodium  hydroxide  is  left  exposed  for  a  long 
time  to  the  air,  it  is  ultimately  all  changed  to  carbonate  be- 
cause the  air  contains  a  minute  amount  of  carbon  dioxide  - 
much  less,  however,  than  it  does  of  water  vapor. 

In  Chapter  IV,  the  experiment  of  absorbing  the  products 


SODIUM  NITRATE  187 

of  burning  a  candle  by  means  of  sticks  of  caustic  soda 
placed  in  a  lamp  chimney  was  described.  It  is  now  obvious 
that  this  absorption  takes  place  because  the  combustion 
products  consist  entirely  of  water  vapor  and  carbon  dioxide. 
Great  quantities  of  sodium  hydroxide  are  used  in  the 
industries  wherever  a  very  strong  alkali  is  required ;  especially 
is  it  used  in  soap  making,  in  petroleum  refining,  and  in 
preparing  coal-tar  products. 

196.  Sodium  peroxide,  Na2O2,  is  made  by  burning  metallic 
sodium  in  the  air.     It  is  itself  a  powerful  oxidizing  agent,  for 
it  contains  twice  as  much  oxygen  as  the  ordinary  oxide, 
Na2O,  and  the  extra  oxygen  is  given  up  with  ease  to  oxidizable 
substances.     Sodium    peroxide    yields    hydrogen    peroxide 
when  it  is  treated  with  an  acid. 

Na2O2  +  H2SO4->Na2SO4  +  H2O2. 

Hydrogen  peroxide  is  useful  as  an  oxidizing  agent  in  general 
and  particularly  as  an  antiseptic  and  as  a  bleaching  agent 
for  silk  and  wool. 

A  convenient  method  for  preparing  oxygen  in  the  labo- 
ratory is  to  treat  sodium  peroxide  with  water.  Probably 
hydrogen  peroxide  is  first  formed  and  then  decomposed  into 
water  and  oxygen  by  the  high  temperature  resulting  from 
the  heat  of  the  reaction. 

197.  Sodium    Nitrate,    NaNO3  —  Chili    Saltpeter.     This 
compound  of  sodium  is  found  in  immense  deposits  in  Chili 
in  South  America.     It  is  a  valuable  fertilizer  —  furnishing 
nitrogen  in  a  soluble  form,  and  hence  readily  available  for 
plant  food.     Sodium  nitrate  is  also  the  raw  material  from 
which  most  of  the  nitric  acid  of  commerce  is  made. 

198.  Flame  Test  for  Sodium.     Sodium  is  remarkable  for 
the  intense  pure  yellow  light  which  the  vapor  of  the  metal 


188 


SODIUM 


FIG.  34.  —  Packing  Sodium  Nitrate  in  Chili. 

or  of  its  compounds  imparts  to  a  flame.  When  a  clean  plati- 
num wire  is  held  in  the  colorless  flame  of  a  Bunsen  burner,  no 
effect  is  noticed.  If  the  wire  is  dipped  into  a  solution  of  so- 
dium chloride  or  any  sodium  compound  and  then  held  in  the 
flame,  the  latter  is  colored  intensely  yellow.  The  minutest 
trace  of  sodium  causes  this  coloration,  and  so  this  flame  test  is 
valuable  as  a  means  of  detecting  the  presence  of  this  element. 

199.  Potassium  is  an  element  which  is  extremely  like 
sodium.     In  fact,  all  that  we  have  said  about  sodium  would 
apply   almost   equally   well    for   potassium.     Potassium   is 
even  a  little  more  active  than  sodium ;  it  decomposes  water 
with  even  more  violence. 

200.  Natural  Occurrence  of  Sodium  and  Potassium.     All 
of  the  simple  compounds  of  both  sodium  and  potassium  are 
easily  soluble  in  water.     Hence  sodium  chloride  is  found  in 


GLASS 


189 


such  abundance  in  sea  water.  The  rain  washes  it  from  the 
land  into  the  sea.  Sea  water  contains  potassium  salts  also 
but  in  much  smaller  amount.  Many  rocks  contain  both 
sodium  and  potassium.  For  example,  the  minerals  feldspar 
and  mica  contain  either  one  or  both  of  these  metals  in  the 
form  of  double  sili- 
cates with  alumin- 
ium. As  the  rocks 
slowly  disintegrate 
under  the  action  of 
the  frost,  the  rain, 
and  the  carbon 
dioxide  of  the  air, 
the  potassium  and 
sodium  become 


FIG.  35.  —  Results  of  Using  Potassium  Salts  as 
Fertilizer.  A,  fertilized  with  200  Ib.  Muriate  of 
Potash  per  acre ;  yield  per  acre,  73.4  bushels. 
B,  without  fertilizer ;  yield  per  acre,  32.1  bushels. 
These  experiments  were  conducted  side  by  side  on 
soil  containing  otherwise  sufficient  plant  food. 


washed  out  as  solu- 
ble carbonates.  On  percolating  through  the  soil  these  salts 
become  changed  before  long  into  nitrates  or  chlorides  of  the 
metals.  Potassium  salts  are  essential  to  the  growth  of 
plants,  and  much  of  the  potassium  thus  washed  from  the  dis- 
integrated rocks  is  taken  from  the  soil  by  the  roots  of  plants. 
On  the  other  hand,  the  greater  part  of  the  sodium  is  washed 
on  into  the  ocean. 

201.  Glass.  Sodium  and  potassium  are  essential  con- 
stituents of  glass,  which  is  a  double  silicate  of  either  sodium  or 
potassium  on  the  one  hand  and  of  calcium  Or  lead  on  the  other 
hand.  Common  window  glass,  for  example,  is  a  sodium 
and  calcium  silicate  of  approximately  the  composition, 


Glass  is  comparatively  easy  to  melt  as  compared  with  most 
natural  silicates,  and  when  it  cools,  it  remains  transparent 


190  SODIUM 

while  it  hardens.  Like  feldspar,  mica,  and  other  natural 
silicates,  glass  is  almost  insoluble  in  water,  but  it  must  be 
remembered  that  glass,  too,  like  them,  is  a  complex  com- 
pound, whereas  all  of  the  simple  compounds  of  both  sodium 
and  potassium  are  comparatively  very  soluble. 

SUMMARY 

Common  salt  is  composed  of  chlorine  and  sodium,  the  former  a 
non-metal,  the  latter  a  metal. 

Metallic  sodium  is  obtained  by  the  electrolysis  of  sodium  chloride, 
or  better,  by  electrolysis  of  sodium  hydroxide. 

Properties  of  Sodium.  Sodium  differs  from  the  more  common 
metals  in  that  it  is  far  more  active.  It  unites  with  all  non- 
metals.  It  displaces  hydrogen  from  water  and  metals  from 
their  compounds.  Sodium  is  very  soft ;  it  is  light  in  weight, 
and  it  melts  below  the  boiling  point  of  water.  It  possesses, 
however,  metallic  luster  and  conductance  for  electricity, 
which  are  properties  that  belong  to  all  metals. 

Important  compounds  of  sodium  are:  sodium  chloride,  NaCl,  a 
neutral  salt ;  sodium  bicarbonate,  NaHCO8,  a  compound  which 
finds  use  in  neutralizing  acids  and  in  the  production  of  carbon 
dioxide;  sodium  carbonate,  Na2C03,  a  salt  which  is  mildly 
alkaline  and  used  as  a  washing  powder ;  sodium  hydroxide, 
NaOH,  which  is  very  strongly  alkaline;  sodium  peroxide, 
Na202,  which  is  a  strong  oxidizing  agent  and  a  source  of 
hydrogen  peroxide;  and  sodium  nitrate,  NaN03,  a  valuable 
fertilizer  and  source  of  nitric  acid.  Baking  powders  usually 
contain  sodium  bicarbonate,  together  with  some  mildly  acid 
solid  substance  such  as  cream  of  tartar. 

Potassium  is  a  metallic  element  extremely  like  sodium  in  properties. 
It  is  more  active  than  sodium. 

All  simple  compounds  of  sodium  and  potassium  are  easily  soluble 
in  water.  These  elements,  however,  are  essential  constitu- 
ents of  glass  and  of  minerals  like  feldspar  and  mica,  which 
are  practically  insoluble,  but  in  many  years  the  slow  weather- 
ing of  rocks  does  wash  out  the  salts  of  sodium  and  potassium. 


QUESTIONS  191 

Questions 

1.  Why  is  common  salt  rather  than  some  other  naturally  oc- 
curring compound  used  to  prepare  sodium  compounds? 

2.  Explain  the  use  of  baking  soda  in  connection  with  sour  milk 
in  raising  dough. 

3.  Which  would  be  more  economical  to  buy,    washing  soda 
that  had  stood  in  an  open  barrel  for  a  month,  or  freshly  opened 
washing  soda  ?     Why  ? 

4.  Why  are  soda  crystals  regarded  as  a  compound  rather  than 
as  a  mixture  of  sodium  carbonate  and  water? 

5.  How  could  you  make  caustic  soda,  using  washing  soda  as 
the  source  of  the  sodium? 

6.  How  is  metallic  sodium  protected  from  the  action  of  the  air? 

7.  What  changes  are  seen  when  a  bit  of  sodium  is  left  for  a  long 
time  exposed  to  the  air  ?     Give  equations  for  chemical  changes  and 
explain  the  physical  changes  involved. 

8.  In  a  hollow  in  a  large  rock  near  the  ocean,  some  small  cube- 
shaped  crystals  were  found.     What  substance  had  probably  crys- 
tallized there  ? 

9.  What  substance  studied  in  this  chapter  might  be  used  to 
dry  gases  ? 

10.  Why  is  sodium  carbonate  used  to  scrub  dirty  floors?     Why  is 
sodium  hydroxide  not  used  instead  ? 

11.  Why  is  sodium  bicarbonate  to  be  preferred  to  sodium  car- 
bonate for  use  in  fire  extinguishers  ? 

12.  What  weight  of  metallic  aluminium  could  be  prepared  by 
using  1  kilogram  of  sodium  according  to  the  reaction: 

3  Na  +  AlCl3->3  NaCl  +  Al? 

13.  Glass  is  made  by  melting  together  sodium  carbonate,  lime- 
stone, CaCO3,  and  pure  sand,  Si02,  the  reaction  being  approximately 

Na2C03  +  CaC03  +  6  Si02->  Na20  -  CaO  •  6  Si02  +  2  CO2. 
What  weights  of  materials  would  be  taken  to  make  1000  kilograms 
of  glass? 

14.  How  many  cubic  meters  of  carbon  dioxide  would  escape  from 
the  melting  pot  in  Question  13  ? 

B.  AND  W.  CHEM. 13 


CHAPTER   XVIII 

CALCIUM 

SODIUM  and  potassium  in  the  uncombined  or  metallic 
state  are  unfamiliar  in  everyday  life,  but  in  their  compounds 
these  elements  are  abundant  and  necessary  to  the  welfare  of 
mankind.  Calcium  is  a  metal  that  is  like  sodium  and  potas- 
sium in  these  respects  as  well  as  in  many  others.  It  is  never 
found  uncombined  in  nature,  and  in  the  metallic  form  it  is 
not  much  more  than  a  laboratory  curiosity. 

Calcium  salts  are  found  in  sea  water  in  very  small  quantity ; 
they  are  found,  too,  in  appreciable  although  small  quantities 
in  spring  and  river  waters,  especially  those  that  are  known  as 
hard  waters.  Calcium  is  an  even  more  abundant  constit- 
uent of  the  rocks  of  the  earth's  surface  than  sodium  or  potas- 
sium. Limestone  and  marble,  which  are  both  calcium  car- 
bonate, CaCO3,  gypsum,  which  is  hydrated  calcium  sulphate, 
CaSO4  •  2  H2O,  and  phosphate  rock,  which  is  mainly  calcium 
phosphate,  Ca3(PO4)2  are  our  most  important  natural  com- 
pounds of  calcium. 

202.  Preparation  of  Calcium.     Metallic  calcium  can  be 
obtained  in  much  the  same  manner  as  metallic  sodium.     The 
electric  current  is  passed  through  molten  calcium  chloride, 
which  melts  more  easily  than  sodium  chloride,  and  the  metal 
is  set  free  at  the  negative  pole  while  the  chlorine  as  usual  is 
liberated  at  the  positive  pole. 

203.  Physical    Properties.     Metallic    calcium    is    silver- 
white,  and  like  all  metals  it  has  a  metallic  luster  and  conducts 

192 


CALCIUM  CHLORIDE  193 

electricity.  It  is  harder,  heavier,  and  tougher  than  sodium. 
It  is  about  as  hard  as  zinc ;  it  is  about  one  and  one  half  times 
as  heavy  as  water  and  thus  it  is  light  in  comparison  with  iron, 
zinc,  or  copper;  it  is  somewhat  malleable  and  extremely 
tough. 

204.  Chemical  Properties.     Calcium  is  less  active  than 
sodium  or  potassium,  but  it  is  still  an  extremely  active  metal. 
This  accounts  for  its  never  being  found  free  in  nature.     Cal- 
cium is  not  much  attacked  by  dry  air  at  the  ordinary  tem- 
perature ;  in  moist  air,  however,  it  is  attacked  quite  rapidly. 
When  heated,  it  catches  fire  and  burns  with  intense  heat, 
forming,  in  the  main,  the  oxide  CaO,  but  there  is  always  a 
little  nitride  formed  as  well.     When  heated  in  pure  nitrogen, 
calcium  forms  the  nitride  very  readily.     This  nitride,  how- 
ever, will  catch  fire  when  heated  in  air  and  burn  to  the  oxide. 

Calcium  reacts  with  cold  water  and  liberates  hydrogen : 

Ca  +  2  HOH-^Ca(OH)2  +  H2. 

This  reaction  is  far  less  violent  than  the  similar  action  of 
sodium,  which,  of  course,  is  to  be  expected  if  calcium  is  a  less 
active  metal  than  sodium.  Furthermore,  calcium  hydroxide 
is  rather  insoluble  and  is  inclined  to  adhere  to  the  metal 
surface,  thus  impeding  the  reaction  with  the  water.  Like 
sodium,  calcium  combines  with  all  the  non-metallic  elements, 
but  with  considerably  less  violence  in  each  case. 

COMPOUNDS  OF  CALCIUM 

205.  Calcium  Chloride,   CaCl2,  is   an  extremely  soluble 
substance.     It  is  left  in  solution  in  the  carbon  dioxide  gen- 
erator in  which  hydrochloric  acid  acts  on  calcium  carbonate. 
Calcium  chloride  is  not  found  extensively  in  nature.     A 
very  small  amount  is  found  in  sea  water ;  but  large  quantities 


194  CALCIUM 

of  it  are  obtained  in  chemical  factories  as  by-products,  just 
as  it  is  left  as  a  by-product  in  the  generation  of  carbon  dioxide. 
One  of  the  chief  uses  of  calcium  chloride  is  in  non-freezing 
solutions.  It  is,  as  stated,  extremely  soluble  in  water  and  a 
concentrated  solution  of  it  freezes  at  a  temperature  far  below 
the  freezing  point  of  pure  water.  Hence  calcium  chloride 
solution  is  much  used  as  a  liquid  to  circulate  in  the  cooling 
pipes  of  refrigerating  plants. 

206.  Deliquescence.  Calcium  chloride  is  not  only  very 
soluble  in  water  but  it  has  a  strong  attraction  for  water.  To 
illustrate  this  point,  leave  a  few  small  lumps  of  calcium  chic- 
ride  exposed  to  the  air  on  a  watch  glass.  Soon  the  lumps  are 
seen  to  be  growing  moist,  and  before  long  they  have  liquefied 


FIG.  36.  —  Calcium  Chloride  Drying  Tube. 

altogether.  They  have  absorbed  water  vapor  from  the  air 
and  dissolved  in  the  water  so  condensed.  Such  action  is 
called  deliquescence.  This  property  suggests  an  important 
laboratory  use  for  calcium  chloride,  namely,  to  dry  gases. 
Some  of  this  substance  in  granulated  form  (A)  is  placed  in 
the  center  of  a  wide  tube  (drying  tube)  between  two  plugs  of 
cotton  (B)  to  keep  it  in  place  (Fig.  36).  The  gas  to  be  dried, 
for  example  the  hydrogen  generated  by  the  action  of  zinc 
on  hydrochloric  acid  solution,  and  consequently  nearly  sat- 
urated with  water  vapor,  is  passed  through  this  tube  of 
calcium  chloride  and  emerges  practically  free  of  water 
vapor. 


CALCIUM  CARBONATE 


195 


207.  Calcium  Carbonate,  CaCO3.  This  substance  is 
found  in  nature  in  rocks  of  widely  differing  appearance. 
Iceland  spar  is  the 
purest  form.  This 
is  crystalline  and  as 
transparent  as  glass. 

Marble  is  a  very 
pure  form,  and  it  is 
beautifully  white  in 
consequence  of  its 
being  composed  of 
a  mass  of  tiny  crys- 
tals cemented  to- 
gether. The  mass 
of  crystals  presents 
a  dazzling  white 
appearance  for  the 
same  reason  that  snow  does,  snow  consisting  of  a  lot  of  tiny 
ice  crystals.  A  compact  block  of  ice  is  colorless  and  trans- 
parent like  Iceland  spar. 

Limestone  is  the  most  abundant,  although  the  least  at- 
tractive, form  of  calcium  carbonate.  It  shows  less  marked 
crystalline  structure  than  marble,  and  it  is  usually  of  a  dirty 
gray  color. 

The  origin  of  limestone  from  the  shells  of  sea  animals  of  a 
previous  age  has  been  discussed  under  the  subject  of  carbon 
dioxide.  Limestone  serves  as  the  chief  source  of  calcium 
compounds  as  well  as  an  important  source  of  carbon  dioxide, 
for  when  it  is  heated,  oxide  of  calcium  is  left  as  a  solid,  while 
the  carbon  dioxide  escapes  : 


FIG.  37.  —  Iceland  Spar, 


CO2  \. 


196  CALCIUM 

208.  Calcium  Oxide,  or  Quicklime,  CaO.  The  decomposi- 
tion of  calcium  carbonate  by  heat  may  be  shown  on  a  small 
scale  if  a  little  powdered  marble  is  heated  in  a  crucible  in 
a  Bunsen  flame.  That  carbon  dioxide  is  given  off  may  be 
shown  by  introducing  a  drop  of  limewater  suspended  on  a 
glass  rod  into  the  crucible.  This  is  at  once  clouded  and  much 
more  strongly  than  it  would  be  by  the  gases  coming  from  the 
flame  alone.  It  is  clouded  but  slowly  when  held  in  the  same 
way  in  an  empty  crucible  which  is  heated  over  a  similar  flame. 

The  residue  is  still  a  powder,  but  it  no  longer  shows  the 
glint  of  the  crystal  grains  of  the  marble ;  it  has  a  uniformly 
dull  appearance.  That  it  is  different  chemically  from  the 
original  marble  may  be  shown  by  barely  wetting  it  with  a 
few  drops  of  water.  If  one  observes  carefully,  it  is  noticed 
that  considerable  heat  is  produced.  The  effect  may  be  better 
observed  if  a  large  lump  of  quicklime,  that  is,  the  commercial 
calcium  oxide  obtained  by  heating  limestone  in  kilns,  is  dipped 
into  water  and  then  taken  out.  The  lime  is  porous,  and 
soaks  up  a  good  deal  of  water.  Soon  it  is  noticed  that  the 
lump  is  growing  warm  and  a  little  later  clouds  of  steam 
issue  and  the  lump  cracks  and  expands  and  finally  falls 
to  a  fluffy  white  powder. 

If  some  of  this  powder,  or  the  powder  left  in  the  crucible, 
is  wet  with  a  little  more  water  and  placed  on  a  piece  of  lit- 
mus paper,  it  colors  the  latter  blue.  This  shows  that  the 
substance  is  alkaline.  Now  marble  does  not  interact  at  all 
with  water,  and  it  has  not  the  property  of  turning  litmus  blue. 

The  burning  of  limestone  to  make  quicklime  is  an  enor- 
mous industry,  and  rows  of  lime  kilns  are  a  familiar  sight  in 
the  regions  of  limestone  quarries.  Large  lumps  of  limestone 
are  packed  in  the  kilns,  where  they  are  heated  for  a  number 
of  hours  either  by  being  mixed  with  the  burning  fuel  or, 


CALCIUM   HYDROXIDE,  OR  SLAKED   LIME 


197 


limestone 


better,  by  being  subjected  to  the  heat  of  the  hot  gases  from 
the  fire.  The  product  is 
the  quicklime  of  com- 
merce. It  is  usually 
packed  in  barrels  for 
shipment,  but  it  is  very 
important  that  the  bar- 
rels be  kept  dry.  If  the 
lime  gets  wet,  not  only 
is  its  usefulness  impaired 
but  there  is  grave  danger 
of  fire,  for  the  reaction 
with  water  may  develop 
heat  enough  to  set  the 
wood  of  the  barrel  on  fire. 
Calcium  oxide  is  very 
infusible,  and  thus  lime 
finds  a  use  in  making 
crucibles  and  in  lining 
furnaces  for  very  high  tern- 
perature  work.  Its  use  1>F'°:  38'T*  "?•.  «?"•  ,.Thot  TCrtic,a' 

shait  is  packed  with  broken  limestone.     At 
in  the    SO-called   limelight    regular  intervals,  the  charge  is  replenished 

is  particularly  interesting.   with  fresh  ^stone  at  the  top,  as  the 

quicklime  is  taken  out  from  the  bottom. 

Here  the  flame  of  an  oxy- 

hydrogen  blowpipe  is  directed  upon  a  block  of  lime  which  be- 
comes intensely  hot  without  melting  and  thus  emits  a  brilliant 
white  light. 

209.   Calcium  Hydroxide,  or  Slaked  Lime,  Ca(OH)2.     The 
reaction  of  quicklime  with  water, 

CaO  +  H20->Ca(OH)2, 
yields  calcium  hydroxide,  which  is  ordinarily  known  as  slaked 


198  CALCIUM 

lime.  Calcium  hydroxide  is  slightly  soluble  in  water,  and 
as  we  have  seen,  the  solution  colors  litmus  blue,  showing 
that  it  is  alkaline.  When  slaked  lime  is  stirred  with  water 
a  saturated  solution  of  calcium  hydroxide  is  obtained,  and 
if  then  the  undissolved  substance  is  allowed  to  settle,  the 
clear  solution  may  be  siphoned  off.  This  is  known  as  lime- 
water.  Limewater  is  very  useful  on  account  of  its  mild 
alkalinity,  and  it  is  frequently  mixed  with  the  food  of  infants 
and  invalids  to  counteract  a  sour  or  acid  condition  of  the 
stomach.  The  use  of  lime  water  in  testing  for  carbon  dioxide 
has  already  been  mentioned. 

When  slaked  lime  is  agitated  with  water,  only  a  little  of 
the  calcium  hydroxide  actually  dissolves;  the  rest,  which 
is  very  light  and  flocculent,  becomes  suspended  in  the  liquid. 
Such  a  suspension  is  known  as  milk  of  lime  because  it  has 
the  appearance  of  milk.  Common  whitewash  is  nothing 
more  than  milk  of  lime. 

Slaked  lime  finds,  perhaps,  its  largest  application  in'making 
mortar  and  plaster.  Lime  is  mixed  with  enough  water  to 
slake  it  and  leave  it  as  a  thin  paste.  Sand  is  then  put  in  to 
give  it  body,  and  often  hair  is  added  in  order  to  sustain  the 
weight  of  the  plaster  until  it  hardens.  After  the  plastic 
mass  has  been  put  in  place  with  a  trowel,  it  soon  sets  or 
hardens  and  makes  a  permanent  part  of  the  masonry  or  of 
the  plastering  of  houses.  The  setting  is  due  first  to  the 
drying  out  of  the  surplus  water  of  the  mixture,  but  the 
permanent  setting  is  largely  caused  by  a  chemical  reaction 
with  the  carbon  dioxide  of  the  air, 

Ca(OH)2  +  CO2->CaCO3  +  H2O, 

by  which  calcium  carbonate  is  formed.  Thus  the  plaster 
finally  becomes  chemically  the  same  substance  as  the  lime- 


CALCIUM   BICARBONATE  199 

stone  from  which  it  was  originally  derived.  In  consequence 
of  the  small  amount  of  carbon  dioxide  in  the  air  and  the 
slowness  with  which  it  can  penetrate  the  mass  of  the  plaster, 
the  change  from  calcium  hydroxide  to  calcium  carbonate  is 
almost  never  complete,  the  internal  portions  of  the  plaster 
remaining  unchanged  for  many  years.  The  water  that  is 
liberated  during  the  setting  of  plaster  is  largely  responsible 
for  the  dampness  of  the  air  in  new  buildings.  Sometimes 
the  setting  of  plaster  is  hastened  by  burning  wood  in  stoves 
in  new  houses ;  the  heat  serves  to  help  circulate  the  air  and 
thus  to  more  rapidly  dry  out  the  plaster,  and  the  carbon 
dioxide  from  the  fire,  if  allowed  to  escape  into  the  air  of  the 
building,  hastens  the  change  of  the  slaked  lime  into  calcium 
carbonate. 

210.  Water-slaked  and  Air-slaked  Lime.     Unless  lime  is 
preserved  in  hermetically  sealed  containers,  a  thing  which 
it  is  practically  impossible  to  do,  lime  must  be  used  before  it 
has  stood  too  long  because  it  is  all  the  time  deteriorating 
through  action  with  the  components  of  the  air.     In  the  first 
place,  it  reacts  with  the  water  vapor  and  slowly  forms  cal- 
cium hydroxide ;   then  the  calcium  hydroxide  reacts  with  the 
carbon  dioxide  and  ultimately  changes  to  calcium  carbonate. 
Lime  which  has  thus  been  kept  too  long  has  become  air 
slaked.     It  then  refuses  to  slake  properly  with  water ;  that 
is,  it  fails  to  heat  up  strongly  and  fall  to  the  powdery  calcium 
hydroxide  that  we  know  as  water-slaked  lime. 

211.  Calcium   Bicarbonate.     A   very   interesting   experi- 
ment which  shows  an  important  property  of  calcium  bicar- 
bonate is  to  blow  the  breath  from  the  lungs  through  a  glass 
tube  into  a  test  tube  of  lime  water.     As  already  seen  (page 
52),  a  turbidity  is  quickly  produced,  due  to  a  precipitate  of 
calcium  carbonate  formed  from  the  carbon  dioxide  of  the 


200 


CALCIUM 


breath  and  the  calcium  hydroxide  of  the  limewater.  But 
on  persistently  blowing  the  breath  into  the  limewater,  a 
surprising  thing  occurs;  the  turbidity  disappears  and  we 
again  obtain  a  clear  solution. 

This  result  is  due  to  the  formation  of  calcium  bicarbonate, 
Ca(HCO3)2,  a  substance  which  is  more  soluble  than  calcium 
carbonate.  After  the  calcium  carbonate  has  all  precipitated, 
the  excess  of  carbon  dioxide  forms  a  weak  solution  of  carbonic 
acid,  and  this  reacts  with  the  calcium  carbonate : 

H2CO3  +  CaCO3->Ca(HC03)2. 

An  effect  that  is  often  seen  on  the  marble  shelves  of  soda 
fountains  may  be  accounted  for  in  the  same  way ;  namely, 

where  the  carbon- 
ated water  drips 
upon  the  marble  a 
hollow  is  in  time 
eaten  away,  due  to 
the  solvent  action  of 
the  carbonic  acid 
and  the  formation 
of  calcium  bicar- 
bonate. 

A  similar  phe- 
nomenon occurs  in 
limestone  regions  in 
the  earth.  Water 
which  percolates 
through  the  surface 
soil  in  wooded  re- 
gions becomes  nearly 

FlG.  39.  —  Stalactites  and  Stalagmites  in  Mammoth 

Cave  saturated  with  car- 


HARD  WATER  201 

bon  dioxide  from  the  decaying  vegetation;  and  when  it 
flows  through  limestone,  it  slowly  dissolves  away  the  rock. 
Thus  in  the  course  of  ages  immense  caverns  are  sometimes 
hollowed  out.  The  famous  Mammoth  Cave  of  Kentucky 
is  supposed  to  have  been  formed  in  this  way. 

212.  Calcium  Sulphate.     This  salt  of  calcium  occurs  in 
nature  in  the  hydrated  form  as  the  mineral  gypsum,  CaSO4- 
2  H2O.     Alabaster,  which  is  used  for  fine  vases  and  other 
ornamental  purposes,  is  one  of  the  natural  forms  of  hydrated 
calcium  sulphate.     Calcium  sulphate  is  also  present  in  hard 
waters  and  in  sea  water,  although  it  is  but  sparingly  soluble. 
Plaster  of  Paris  is  made  by  partially  dehydrating  gypsum. 

2  CaSO4  •  2  H2O  ->  3  H2O  f  +  (CaSO4)2  •  H2O. 

The  reverse  reaction  takes  place  when  the  plaster  is  mixed 
with  water  and  "  setting  "  of  the  plaster  results.  The  set- 
ting of  cement,  although  much  more  complicated  chemically 
than  the  setting  of  plaster  of  Paris,  consists  in  the  main  in  a 
similar  addition  of  water  to  the  finely  powdered  anhydrous 
material. 

213.  Hard   Water.     Water   which   contains   calcium   bi- 
carbonate, or  calcium  sulphate,  or  in  fact  any  soluble  salt  of 
calcium  or  magnesium,  is  said  to  be  hard.     Such  water  does 
not  readily  yield  lather  with  soap,  and  if  it  is  used  in  steam 
boilers,  a  deposit  of  mineral  matter  known  as  boiler  scale  is 
formed  inside  of  the  tubes  to  the  great  injury  of  the  boilers. 

214.  Softening  Hard  Water.     There  are  a  good  many 
kinds  of  soap,  but  for  the  purpose  of  discussing  its  action 
with    hard  water,   we   may  regard    soap   as    consisting  of 
sodium    stearate,    NaCi8H35O2.     Now    sodium  stearate    is 
soluble   in    water,  but   calcium   and   magnesium   stearates 
are  not  soluble.     Therefore  when  soap  is  used  with  hard 


202 


CALCIUM 


water  a  precipitate  of  calcium  or  magnesium  stearate  is 
thrown  down : 

2  NaCi8H35O2  +  Ca(HCO3)2  -*  Ca(Ci8H3502)2 |  +  2  NaHC03. 

Any  one  who  has  tried  to  wash  his  hands  with  hard  water 
and  soap  knows  of  the  dirty  sediment  which  collects  in  the 
wash  basin. 

Of  course  the  amount  of  calcium  and  magnesium  com- 
pounds in  hard  water  is  limited,  and  enough  soap  can  be 
taken  to  react  with  all  the  hardness.  After  this,  any  addi- 


FlG.  40.  —  Settling  Tanks,  Water  Softening  Process. 

tional  soap  forms  lather  freely.  In  laundries  where  a  great 
deal  of  water  is  used,  it  would  become  a  matter  of  consider- 
able expense  to  depend  upon  soap  to  soften  the  water  in  this 
way. 

It  becomes  necessary,  therefore,  to  soften  hard  waters 
before  they  can  be  used  in  laundries  or  in  steam  boilers.  If, 
as  is  very  often  the  case,  the  hardness  is  caused  by  calcium 
bicarbonate  alone,  the  method  of  softening  is  comparatively 


VALENCE  203 

simple.  Calcium  bicarbonate  is  very  unstable,  and  is  en- 
tirely broken  up  at  the  boiling  temperature  of  water : 
Ca(HCO3)2  ->CaCO3 1  +  H2O  +  CO2f.  So  if  the  water  is 
first  heated,  all  of  the  calcium  will  be  thrown  down  as  in- 
soluble calcium  carbonate,  and  the  water,  which  is  now  soft, 
may  be  used  in  the  laundries  or  pumped  into  steam  boilers. 
Hardness  caused  by  calcium  bicarbonate  is  known  as  tem- 
porary hardness  because  it  can  be  so  easily  overcome  by 
heating. 

Permanent  hardness  can  be  remedied  by  adding  chemical 
reagents  which  will  precipitate  the  calcium  and  magnesium 
compounds.  For  example,  if  the  hardness  is  caused  by  cal- 
cium sulphate,  the  addition  of  sodium  carbonate  will  pre- 
cipitate calcium  carbonate,  and  leave  the  soluble  but  harm- 
less sodium  sulphate  in  solution  : 

CaSO4  f  Na2CO,  ->  CaCO3 1  +  Na2SO4. 

This  method  is  largely  used  in  practice. 

Either  type  of  hardness  can  be  removed  by  distillation, 
the  involatile  mineral  substances  remaining  behind  when 
the  water  evaporates.  This  last  method,  however,  is  too 
costly  for  extensive  use,  and  is  only  employed  when  limited 
amounts  of  very  pure  water  are  necessary. 

215.  Valence.  The  pupil  has  doubtless  wondered  why 
the  formula  of  sodium  chloride  is  written  NaCl  and  that  of 
calcium  chloride  CaCl2;  that  of  water  *H2O  and  that  of 
hydrogen  chloride  HC1.  Formulas,  as  we  learned  in  Chapter 
XV,  are  used  not  only  to  designate  substances,  but  to  show 
their  composition.  The  above  formulas  tell  us,  then,  that 
one  atom  of  sodium  is  combined  with  a  single  atom  of  chlo- 
rine, whereas  one  atom  of  calcium  is  combined  with  two  atoms 
of  chlorine ;  that  an  atom  of  oxygen  is  combined  with  two 


204  CALCIUM 

atoms  of  hydrogen,  whereas  an  atom  of  chlorine  is  combined 
with  but  a  single  atom  of  hydrogen. 

Clearly  the  number  of  atoms  of  another  element  that  one 
atom  of  a  given  element  holds  in  combination  is  a  very  im- 
portant property ;  this  property  has  been  given  the  name  of 
valence.  The  valence  of  hydrogen  in  its  compounds  has  been 
chosen  as  one  for  a  standard  of  valence.  The  valence  of 
chlorine  in  hydrogen  chloride  is  one  because  one  atom  of 
chlorine  is  combined  with  a  single  atom  of  hydrogen.  The 
valence  of  oxygen  in  water  is  two  because  the  atom  of  oxygen 
is  combined  with  two  atoms  of  hydrogen.  The  valence  of 
sodium  in  sodium  chloride  is  one  because  one  atom  of  sodium 
is  combined  with  the  same  amount  of  chlorine  that  can 
combine  with  one  atom  of  hydrogen.  The  valence  of  cal- 
cium in  calcium  chloride  is  two  because  an  atom  of  calcium 
is  combined  with  the  same  number  of  atoms  of  chlorine  that 
can  combine  with  two  atoms  of  hydrogen. 

It  is  to  be  noted  that  valence  is  concerned  solely  with  the 
number  of  atoms  that  an  atom  of  a  given  element  holds  in 
combination.  It  has  nothing  whatever  to  do  with  whether 
the  other  elements  are  held  tenaciously  or  feebly. 

SUMMARY 

Calcium  resembles  sodium  and  potassium  in  many  respects.  It  is 
never  found  in  nature  as  the  uncombined  metal,  but  its  com- 
pounds are  very  abundant  and  useful  to  mankind.  The  metal 
may  be  obtained  by  electrolyzing  melted  calcium  chloride. 

Properties  of  Calcium.  Calcium  is  a  very  active  metallic  element, 
but  it  is  less  active  than  sodium  or  potassium.  It  combines 
with  vigor  with  all  non-metals.  It  displaces  hydrogen  from 
water,  although  not  as  vigorously  as  sodium  or  potassium. 
Metallic  calcium  is  about  as  hard  as  zinc.  It  is  heavier  than 
sodium,  but  lighter  than  most  metals.  It  possesses  metallic 
luster  and  conducts  electricity. 


QUESTIONS  205 

Important  compounds  of  calcium  are  calcium  carbonate,  CaCOs,  of 
which  some  of  the  rock  of  the  earth's  surface  consists ;  cal- 
cium oxide  or  quicklime,  CaO,  which  is  obtained  by  strongly 
heating  calcium  carbonate;  calcium  hydroxide  or  slaked 
lime,  Ca(OH)2,  which  is  obtained  by  the  action  of  calcium 
oxide  with  water. 

Quicklime  is  used  for  making  slaked  lime  in  the  preparation  of 
mortar  and  plaster.  The  permanent  hardening  of  these 
materials  depends  upon  their  combining  with  the  carbon  di- 
oxide in  the  air,  thus  forming  calcium  carbonate. 

Calcium  carbonate  reacts  with  carbonic  acid  to  form  calcium  bi- 
carbonate, which  is  somewhat  soluble.  In  nature,  water  con- 
taining carbonic  acid  slowly  dissolves  limestone  rock  and  forms 
caverns. 

Calcium  sulphate  in  a  hydrated  form  occurs  in  nature  as  gypsum. 
Plaster  of  Paris  is"  made  by  partially  dehydrating  gypsum. 

Hard  Waters.  Water  containing  calcium  bicarbonate  or  other 
soluble  calcium  or  magnesium  salts  is  known  as  hard  water, 
and  it  cannot  be  satisfactorily  used  in  laundries  or  in  steam 
boilers.  Temporary  hardness,  which  is  caused  by  calcium 
bicarbonate  alone,  may  be  remedied  by  pre-heating.  Per- 
manent hardness  can  be  relieved  by  chemical  treatment  or  by 
distillation. 

The  valence  of  an  element  is  a  property  which  depends  on  the  num- 
ber of  atoms  of  another  element  which  its  own  atom  may  hold 
in  combination.  The  valence  of  calcium  in  its  compounds  is 
two. 

Questions 

1.  Why  cannot  metallic  calcium  exist  free  in  nature? 

2.  What  occurs  when  marble  is  strongly  heated? 

3.  How  may  lime  water  be  prepared,  starting  with  marble? 

4.  Why  does  a  wood  fire  in  a  new  house  aid  the  setting  of  the 
plaster? 

6.   Explain  the  chemistry  of  limestone  cave  formation. 

6.  Can  you  offer  an  explanation  of  the  formation  of  stalactites 
(consult  a  dictionary,  if  you  are  not  familiar  with  the  meaning  of  the 
word)  in  a  limestone  cave  ? 


206  CALCIUM 

7.  How  might  one  soften  a  little  hard  water  in  order  to  obtain 
a  good  lather  with  soap  ? 

8.  What  would  be  a  good  way  to  keep  metallic  calcium  from 
oxidizing  ? 

9.  Mention  two  ways  in  which  sea  water  might  be  softened  for 
use  in  the  boilers  of  a  battleship.     Which  method  would  give  the 
better  drinking  water? 

10.  What  marked  difference  of  behavior  on  exposure  to  air  might 
be  used  to  tell  whether  a  certain  white  solid  was  sodium  carbonate 
or  calcium  chloride? 

11.  A  crystal  of  gypsum  (from  which  plaster  of  Paris  is  made) 
was  heated  moderately  in  a  dry  test  tube.     The  upper  walls  of  the 
tube  became  covered  with  moisture.     The  gypsum  was  dry  when 
first  heated.     Explain. 

12.  What  weight  of  water  would  be  necessary  to  slake  one  kilo- 
gram of  quicklime? 

13.  What  volume  of  carbon  dioxide  must  bev  absorbed  by  one 
kilogram  of  quicklime  before  it  is  completely  air-slaked?     What 
will  be  the  final  weight  of  the  air-slaked  lime? 

14.  Some  hard  water  contains  0.5  gram  of  calcium  sulphate  per 
liter.     How  much  sodium  carbonate  should  be  added  to  1000  liters 
of  the  water  to  soften  it  according  to  the  reaction : 

CaS04  +  NasCOs-^CaCOa.H-  Na2S04? 

15.  If  the  same  water  were  used  for  washing,  what  weight  of 
soap  would  be  used  up  by  each  liter  of  the  water  before  a  lather 
could  be  produced?     (See  Sec.  214,  and  use  an  equation  similar 
to  that  there  given.) 

16.  Starting  on  the  basis  of  the  valence  of  hydrogen  as  one,  what 
is  the  valence  of  each  of  the  elements  in  the  following  compounds :  1 

HC1,  HBr,  H2S,  NH3,  CH4; 

H20,  CaO,  Sn02,  S03,  OsO4,  Fe203,  N205,  Mn207  ; 

HC1,  AgCl,  CuCl2,  FeCl2,  FeCl3,  SnCl2,  SnCl4,  PC13,  PC15. 

H2O,  CaO,  CaS,  Ca3N2. 


CHAPTER  XIX 

ACIDS  AND  BASES;    NEUTRALIZATION 

216.  Properties  of  Acids.     We  have,  up  to  this  point, 
spoken  of  a  few  acids ;  namely,  hydrochloric  acid,  sulphuric 
acid,  and  the  very  weak  acid,  carbonic  acid,  which  is  obtained 
when  carbon  dioxide  dissolves  in  water.     These  acids,  as 
well  as  other  acids,  have  a  good  many  properties  in  common, 
those  properties  that  we  have  learned  being,  first,  a  sour 
taste,  which  is  very  strong  with  hydrochloric  and  sulphuric 
acids  and  much  weaker  with  carbonic  acid ;   second,  ability 

,  to  change  the  vegetable  coloring  matter  litmus  from  blue 

>  to  red;    and  third,  ability  to  give  off  hydrogen  gas  when 

allowed  to  react  with  zinc,  magnesium,  or  certain  other  active 

metals.     Carbonic  acid  is  weaker  than  the  other  acids,  and 

J  one  ordinarily  fails  to  perceive  any  escape  of  hydrogen  gas 

on  treating  this  acid  with  zinc  or  magnesium. 

217.  Characteristic  Component  of  Acids.     The  one  con- 
stituent which  is  possessed  by  all  acids  is  hydrogen,  but  it  is 
hydrogen  in  a  peculiar  condition  of  combination,  because 
this  hydrogen  is  more  or  less  easily  displaced  by  metals. 
Water,  alcohol,  sugar,  and  petroleum  all  contain  hydrogen, 
but  these  substances  are  not  acids,  for  their  hydrogen  is  not 
easily  displaced  by  metals  like  zinc  and  iron,  neither  do  these 
substances  taste  sour  nor  do  they  affect  the  color  of  litmus. 
The  nature  of  this  peculiar  state  in  which  hydrogen  exists  in 
acids  will  be  taken  up  in  a  later  chapter,  and  now  we  shall 

B.  AND  W.  CHEM. 14  207 


208  ACIDS  AND   BASES;  NEUTRALIZATION 

describe  the  manner  in  which  a  certain  large  class  of  acids 
—  the  so-called  oxygen  acids,  or  oxy-acids  —  is  formed. 

218.  Oxy-acids;    Carbon   Dioxide    and   Carbonic    Acid. 
As  we  have  seen,  carbon  burns  in  air  or  oxygen  to  form 
carbon  dioxide,  CO2,  and    the  carbon  dioxide   dissolves  to 
some  extent  in  water,  forming  the  weak  acid,  carbonic  acid. 
It  is  believed  that  carbonic  acid  is  a  definite  chemical  com- 
pound formed  from  a  molecule  each  of  carbon  dioxide  and 
water:  CO2  +  H2O -^  H2CO3, 

but  this  compound,  H2CO3,  is  so  feebly  joined  together  — 
or  to  use  the  technical  expression,  it  is  so  unstable  —  that 
it  can  only  exist  when  dissolved  in  a  comparatively  large 
amount  of  water.  As  we  know,  when  the  pressure  is  re- 
leased by  removing  the  cork  from  a  bottle  of  carbonated 
water,  bubbles  of  a  gas,  which  is  carbon  dioxide,  commence 
to  escape.  The  compound,  carbonic  acid,  is  breaking  down, 

and  the  reaction     TT  ^^     ^  TT  /^       /^/^ 
.H2ClJ3  —>  rl2(J  -f-  l_,U2 

is  taking  place ;  that  is,  the  reaction  of  formation  is  proceed- 
ing in  the  reverse  direction. 

219.  Sulphur    Dioxide    and  •  Sulphurous    Acid.    When 
sulphur  burns,  a  very  choking  gas  is  formed,  which  is  sulphur 
dioxide,  SO2.     This  gas  dissolves  in  water  much  more  easily 
than  carbon  dioxide,  and  a  more  strongly  acid  solution  is 
produced.     The  definite  compound,  sulphurous  acid,  H2SO3, 
is  supposed  to  be  formed 

S02  +  H2O->H2SO3 

and  to  exist  in  the  solution,  but,  like  carbonic  acid,  it  is 
unstable  and  readily  breaks  down  again 


SULPHUR  TRIOXIDE   AND   SULPHURIC   ACID         209 

except  when  it  is  dissolved  in  a  large  amount  of  water. 
Under  ordinary  pressure  and  at  20°  C.  one  volume  of  water 
will  dissolve  nearly  40  volumes  of  sulphur  dioxide.  The 
solution  so  formed  is  far  more  strongly  acid  than  that  of 
carbon  dioxide,  and  it  smells  strongly  of  sulphur  dioxide  on 
account  of  the  continuous  slow  escape  of  that  gas. 

220.  Sulphur  Trioxide  and  Sulphuric  Acid.  There  is  an- 
other oxide  of  sulphur,  sulphur  trioxide,  SO3,  which  can  be 
prepared  if  sulphur  dioxide  is  allowed  to  combine  with  more 
oxygen  under  the  right  conditions.  These  conditions  will 
be  more  fully  discussed  in  the  chapter  on  sulphur.  Sulphur 
trioxide  is  extremely  soluble  in  water,  —  in  fact,  this  hardly 
expresses  it;  the  sulphur  trioxide  has  a  great  chemical 
affinity  for  water,  as  is  evidenced  by  the  large  amount  of 
heat  developed  when  the  two  substances  come  in  contact. 
Indeed,  the  greatest  caution  must  be  exercised  in  adding 
sulphur  trioxide  to  water  if  one  would  avoid  a  dangerously 
violent  reaction. 


In  this  case,  there  is  no  doubt  of  the  existence  of  the  definite 
compound  H2SO4  ;  for  unlike  carbonic  and  sulphurous  acids, 
it  can  be  prepared  pure.  At  a  low  temperature  it  can  be 
obtained  as  a  crystalline  solid,  but  at  ordinary  temperatures 
it  is  a  thick,  oily  liquid.  It  can  be  proved  by  chemical  anal- 
ysis to  have  the  composition  corresponding  to  the  formula, 
H2SO4.  The  pure  substance  does  not  show  very  marked 
acid  properties,  but  it  mixes  freely  with  water  and  its  fairly 
dilute  solution  is  very  strongly  acid. 

221.  Phosphorus  Pentoxide  and  Phosphoric  Acid.  When 
phosphorus  burns  with  plenty  of  air,  the  oxide  formed  has  the 
composition  P2O5.  This  oxide  combines  with  the  water  as 


210  ACIDS  AND   BASES;  NEUTRALIZATION 

vigorously  as  does  sulphur  trioxide.  It  forms  with  the  water 
several  clearly  defined  compounds,  but  the  most  important 
one  is  the  common  phosphoric  acid,  H3PO4. 

P2O5  +  3H2O->2H3PO4. 

This  compound  can  be  prepared  pure  as  a  crystalline  solid 
which  melts  at  37°  C.  (about  the  body  temperature)  to  a 
sirupy  liquid.  Like  pure  sulphuric  acid,  this  pure  substance 
has  no  marked  acid  properties,  but  it  mixes  easily  with  water, 
and  the  dilute  solution  is  distinctly  acidic,  —  about  as 
strongly  acidic  as  the  solution  of  sulphurous  acid. 

222.  Non-metals  are  Acid-forming  Elements.  We  have 
now  seen  that  the  oxides  of  three  different  elements,  carbon, 
sulphur,  and  phosphorus,  can  combine  with  hydrogen  oxide, 
or  water,  to  form  compounds, 

CO2  +     H2O->     H2CO3 

502  +     H20->    H2SO3 

503  +     H20->    H2S04 

3H2O->2H3PO4 


and  that  these  compounds,  when  dissolved  in  or  mixed  with 
large  additional  amounts  of  water,  show  acid  properties. 

The  three  elements  just  mentioned  are  non-metals.  It 
is  one  of  the  important  chemical  properties  of  non-metallic 
elements  that  their  oxides  are  acid-forming.  Non-metallic 
elements  possess  distinct  physical  as  well  as  distinct  chemical 
properties  ;  for  they  are  poor  conductors  of  heat  and  elec- 
tricity, and  they  do  not  usually  possess  a  metallic  luster. 
Metals  can  be  easily  distinguished  by  their  peculiar  metallic 
luster  and  by  the  ease  with  which  they  conduct  heat  and 
electricity. 

The  non-metallic  oxides  from  which  acids  can  be  formed 


BASE-FORMING   ELEMENTS  211 

by  combination  with  water  are  known  as  acid  anhydrides, 
the  term  anhydride  being  derived  from  Greek  words  meaning 
without  water. 

As  we  know,  hydrogen  is  essential  in  the  make-up  of  an 
acid.  The  anhydride  contains  no  hydrogen,  but  the  water 
does,  although  in  water  this  hydrogen  is  so  closely  bound 
to  the  oxygen  that  it  is  not  readily  displaced  by  metals  like 
zinc  or  magnesium,  nor  does  the  hydrogen  of  water  exhibit 
the  other  characteristics  of  acids,  such  as  the  sour  taste  and 
the  power  to  redden  litmus.  When  the  acid  anhydride  is 
added  to  water,  it  seems  to  partially  appropriate  the  oxygen 
of  the  water,  leaving  the  hydrogen  less  closely  bound.  So 
the  hydrogen  becomes  displaceable  by  metals,  and  at  the  same 
time  it  develops  the  properties  of  sour  taste  and  effect  on 
litmus.  It  is  for  this  reason  that  in  the  formula  of  an  acid 
the  hydrogen  is  written  by  itself,  and  the  group  containing 
the  acid  anhydride  plus  the  oxygen  from  the  water  is  all 
written  together :  H2  •  SO4  instead  of  H2O  •  SO3. 

223.  Metals  are  Base-forming  Elements.     All  elements 
belong  to  one  of  two  classes,  either  to  the  metals  or  the  non- 
metals.     There  is,   however,   no  sharp  boundary  between 
these  classes,  but  rather,  there  is  a  gradual  transition,  and 
some  of  the  elements  in  between  might  at  one  time  be  classed 
as  metals  and  at  another  as  non-metals.     We  know  that  the 
physical  properties  of  metals  are  very  characteristic;    the 
chemical  properties  are  equally  so.     One  *of  the  most  im- 
portant of  the  chemical  properties  is  the  behavior  of  the 
oxides  of  the  metals ;  for  these,  instead  of  forming  acids  like 
thet)xi3es  oHhelibn-metals,  when  they  combine  with  water, 
produce  bases,  which  are  the  exact  opposites  of  acids. 

224.  Sodium    Oxide    and    Sodium    Hydroxide.     When 
sodium  burns  with  a  plentiful  supply  of  air,  sodium  peroxide, 


212  ACIDS  AND   BASES;   NEUTRALIZATION 

Na2O2,  is  formed.  This  substance  has  already  been  men- 
tioned as  a  source  of  hydrogen  peroxide  and  of  oxygen.  But 
in  the  present  connection  we  are  more  interested  in  sodium 
oxide,1  Na2O,  which  can  be  prepared  by  burning  sodium  with 
a  limited  amount  of  oxygen.  This  oxide  of  sodium  reacts 
very  vigorously  with  water,  producing  great  heat,  just  as 
sulphur  trioxide  and  water  produce  great  heat ;  and  a  distinct 
new  substance  —  sodium  hydroxide  —  is  formed : 

Na2O  +  H20-*2NaOH. 

Sodium  hydroxide  is  a  solid  white  substance.  It  will  dis- 
solve in  less  than  its  own  weight  of  water.  Its  solution 
possesses  properties  which  are  almost  the  exact  opposite  of 
the  properties  of  acid  solutions.  It  has  an  alkaline  instead 
of  a  sour  taste ;  in  experimenting  to  find  out  what  this  taste 
is,  only  very  dilute  solutions  should  be  used,  —  not  more 
than  one  part  by  weight  of  sodium  hydroxide  to  1000  parts 
of  water;  it  turns  red  litmus  blue,  and  it  has  the  power  of 
neutralizing  acids. 

These  alkaline  properties  are  characteristic  not  only  of 
a  sodium  hydroxide  solution,  but  of  the  solution  of  any 
soluble  base.  A  base  is  obtained  when  a  metallic  oxide 
combines  with  water,  and  sodium  hydroxide  is  one  of  the 
strongest  bases. 

225.  Neutralization.  When  a  solution  of  sodium  hydroxide 
is  added  little  by  little  to  a  solution  of  any  acid,  —  let  us  say 
sulphuric  acid,  so  as  to  have  a  concrete  example,  — it  is 
found  that  the  acid  properties  grow  less  and  less  marked 
until  at  a  certain  point  they  disappear  altogether ;  addition 

1  This  is  sometimes  called  sodium  monoxide  when  it  is  wished  to 
distinguish  it  from  the  peroxide.  Otherwise,  sodium  oxide  always 
means  this  oxide. 


CALCIUM  OXIDE  AND   CALCIUM  HYDROXIDE        213 

of  any  further  sodium  hydroxide  imparts  the  characteristics 
of  a  base  to  the  whole  solution.  At  the  exact  point  at  which 
the  acid  properties  have  disappeared  and  the  basic  properties 
have  not  begun  to  appear  the  solution  is  neutral,  just  as  pure 
water  is  neutral.  The  process  of  just  destroying  acid  prop- 
erties by  the  careful  addition  of  a  base  is  known  as  neu- 
tralization; likewise  the  reverse  process,  that  of  exactly 
removing  all  basic  properties  by  carefully  adding  an  acid, 
is  neutralization. 

In  the  above  process  of  neutralization  exactly  two  moles 
of  sodium  hydroxide  must  be  added  to  destroy  the  acid 
properties  of  each  mole  of  sulphuric  acid,  or,  in  other  words, 
one  molecule  of  sodium  hydroxide  for  each  acid  hydrogen 
in  a  molecule  of  sulphuric  acid.  If  the  neutralized  solution 
is  evaporated,  it  is  found  that  after  all  the  water  is  driven 
off  a  solid  white  substance  is  left,  which  is  sodium  sulphate. 
The  reaction  of  neutralization  may  be  formulated : 

2  NaOH  +  H2S04  ->  2  H2O  +  Na2SO4. 

226.  Calcium  Oxide  and  Calcium  Hydroxide.  The  metal 
calcium  yields  calcium  oxide  on  burning.  Calcium  oxide  is 
the  very  common  substance  that  is  known  as  quicklime,  and 
it  is,  of  course,  usually  obtained  by  a  cheaper  method  than 
that  of  burning  the  rather  expensive  metallic  calcium. 
Treated  with  water,  calcium  oxide  shows  characteristics 
similar  to  those  of  sodium  oxide ;  for  a  gceat  deal  of  heat  is 
produced  and  a  base  is  formed,  according  to  the  reaction : 

CaO  +  H2O->Ca(OH)2. 

As  we  have  seen  in  the  preceding  chapter,  the  saturated 
solution  of  calcium  hydroxide,  which  is  known  as  limewater, 
possesses  the  same  alkaline  properties  as  sodium  hydroxide 
solution,  but  to  a  smaller  degree,  because  calcium  hydroxide 


214  ACIDS  AND   BASES;  NEUTRALIZATION 

is  so  insoluble  that  the  solution  contains  a  comparatively 
small  amount  of  the  base. 

227.  Other  Basic  Oxides  and  Bases.     Oxides  of  other 
metals  can  yield  bases;  as,  for  example,  magnesium  oxide, 
MgO  +  H2O  ->  Mg(OH)2 ;      zinc     oxide,     ZnO  -f  H2O  -> 
Zn(OH)2 ;   silver  oxide,  Ag2O  +  H2O  ->  2  AgOH ;   but  these 
last-mentioned  bases  are  much  weaker  in  their  basic  prop- 
erties than  sodium  or    calcium    hydroxide.     All    of    these 
bases  are  capable  of  neutralizing   acids  as  shown  in  the 
following  reactions  : 

Ca(OH)2  +  H2S04-^CaS04  +  2  H2O 
Mg(OH)2  +  H2S04->MgS04  +  2  H2O 
Zn(OH)2  +  H2SO4->ZnSO4  +  2  H2O 
2  AgOH  +  H2SO4  ->  Ag2S04  +  2  H2O 

228.  Characteristic  Component  of  Bases.     It  has  already 
been  seen  that  the  displaceable  hydrogen  of  acids  is  the  com- 
ponent responsible  for  the  acid  characteristics.     When  this 
component    is    removed    during    neutralization,    the    acid 
properties  disappear.     It  is  evident  from  the  above  reactions 
that  bases  likewise  have  a  certain  component  which  must 
be  responsible  for  the  basic  properties,  and  this  component 
is  the  OH,  or  hydroxyl  group.     As  the  non-metallic  oxide  is 
known  as  an  acid  anhydride,  so  the  metallic  oxide  from  which 
a  base  is  formed  by  combination  with  water  is  known  as  a 
basic  anhydride.     It  is  evident  that  the  hydroxyl  group  can- 
not have  come  wholly  from  the  basic  anhydride.     It  appears 
as  if  the  basic  anhydride  had  attached  to  itself  a  molecule 
of  water,  CaO  +  H2O  — >  CaO  •  H2O  and  that  this  compound 
had  rearranged  itself  so  that  its  hydrogen  and  oxygen  formed 
two  hydroxyl  groups 

CaO  +  H2O  -+  CaO  -  H2O  -+  Ca(OH)2. 


PRODUCTS  OF  NEUTRALIZATION         215 

229.  Formation  of  Water  in  Neutralization.  In  the  process 
of  neutralization  it  is  the  hydrogen  of  acids  and  the  hydroxyl 
of  bases  which  mutually  destroy  each  other's  properties. 
They  simply  combine  to  form  water 


in  which,  as  we  know,  neither  basic  nor  acidic  properties 
are  manifest.  It  would  appear  as  if  in  water  these  com- 
ponents were  very  closely  bound  together,  whereas  the  hy- 
drogen of  acids  and  the  hydroxyl  of  bases  are  less  closely 
joined  to  the  rest  of  the  molecule  and  are  thus  free  to  mani- 
fest their  specific  properties. 

230.  Formation  of  Salts  during  Neutralization.     The  for- 
mation of  water  during  neutralization  accounts  for  two  of 
the  original  components  of  the  acid  and  base.     The  other 
components,  that  is,  the  metal  of  the  base  and  the  radical 
of  the  acid,  constitute  a  salt  which  in  many  cases  remains 
dissolved  in  the  solution  ;   in  other  cases  in  which  the  par- 
ticular salt  is  insoluble  it  separates  out  at  once  as  a  solid 
precipitate.     In  any  case,  solid  salt  may  be  made  to  separate 
out  if  the  water  —  both  the  original  solvent  water  and  the 
water  produced  in  the  neutralization  —  is  evaporated  off 
by  heat.     The  immediate  and  invariable  product  of  every 
neutralization  is  water,  but  besides  this  there  is  always  a  salt 
which  is  different  for  every  different  acid  and  base.     A  salt 
may  be  defined  as  a  substance  which  is  composed  of  the  metal 
of  some  base  and  the  radical  of  some  acid.     Different  salts 
differ,  of  course,  in  many  respects  among  themselves,  but 
we  shall  find  that  salts,  as  a  class,  have  a  good  many  prop- 
erties in  common. 

231.  Acids  containing  no  Oxygen.     We  have  just   seen 
that  acids  and  bases  may  be  formed  by  the  union  of  non- 


216  ACIDS  AND   BASES;   NEUTRALIZATION 

metallic  oxides  and  metallic  oxides,  respectively,  with  water. 
But  there  are  acids  and  bases  other  than  those  formed  in 
this  way.  We  have  seen  that  the  essential  component  of  an 
acid  is  displaceable  hydrogen,  and  the  essential  component  of 
a  base  is  a  loosely  attached  hydroxyl  group.  Hydrogen 
chloride,  a  compound  already  familiar  to  us,  contains  only 
hydrogen  and  chlorine,  yet  when  it  is  dissolved  in  water, 
its  hydrogen  assumes  the  easily  displaceable  condition  that 
is  characteristic  of  an  acid.  Hydrochloric  acid  is  one  of  the 
strongest  acids.  It  neutralizes  bases  in  just  the  same  manner 
as  all  other  acids ;  for  example,  it  reacts  with  sodium  hy- 
droxide, giving  water  and  sodium  chloride : 

HC1  +  NaOH^H2O  +  NaCl. 

232.   Acid  Radicals.     When  the  neutralization  reactions 
of  the  oxy-acids  are  compared  with  that  of  hydrochloric  acid, 

H2CO3  +  2  NaOH  ->  2  H2O  +  Na2CO3 
H2SO3  +  2  NaOH  ->  2  H2O  +  Na2SO3 
H2S04  +  2  NaOH  ->  2  H2O  +  Na2SO4 
H3PO4  +  3  NaOH  -^  3  H2O  +  Na3PO4 

it  becomes  obvious  that  the  groups  CO3,  SO3,  SO4,  and  PO4 
function  in  the  same  manner  as  the  single  atom  Cl ;  that  is 
to  say,  these  groups  remain  intact  while  the  hydrogen  is 
removed  and  the  metallic  element  takes  its  place.  The 
term  radical  is  used  to  denote  this  part  of  an  acid  which 
remains  unchanged  when  the  acid  is  neutralized,  and  the 
radical  may  consist  of  a  single  element  only ;  as,  for  example, 
chlorine,  or  of  several  elements.  Salts,  of  course,  contain 
the  same  radicals  as  the  acids  from  which  the  salts  are 
derived. 


METALLIC  RADICALS  217 

233.  Ammonium  Hydroxide.     We  have  not  yet  discussed 
ammonia;    ammonia  water,  however,  is  something  familiar 
to  every  one.     It  is  used  in   the  household   for  cleansing 
purposes  because  it  is  a  mild  alkali.     Ammonia  itself  is  a 
gas  containing  only  hydrogen  and  nitrogen,  and  its  formula 
is  NH3.     It  is  thus  in  no  respect  a  metal  oxide,  yet  it  dis- 
solves freely  in  water  and  gives  a  solution  that  shows  the 
characteristics  of  a  base.     This  indicates  that  in  some  way 
the  water  has  been  altered  so  that  loosely  bound  hydroxyl 
groups  now  exist.     The  NH3  molecules  may  be  supposed 
to  have  each  appropriated  one  of  the  H  atoms  of  a  water 
molecule  to  form  the  group  NH4,  which  is  known  as  the 
ammonium  group,  or  radical,  whereby  the  OH  of  water  is 
left  in  the  loosely  bound  condition : 

NH3  +  HOH  -+  (NH3  •  H)OH  ->  NH4  •  OH. 

234.  Metallic  Radicals.     Ammonium  hydroxide  can  be 
used  to  neutralize  an  acid, 

NH4OH  +  HC1  ->  H2O  +  NH4C1 

and  it  is  found  that  the  ammonium  group  remains  intact 
during  the  process  and  that  it  enters  into  the  composition 
of  the  salt,  ammonium  chloride.  A  group  like  this,  which 
plays  the  same  part  in  a  base  or  salt  as  the  atoms  of  single 
metallic  elements,  is  likewise  known  as  a  radical,  — a  metallic 
radical,  in  this  case. 

Every  one  knows  that  ammonia  water  is  not  nearly 
so  caustic  an  alkali  as  sodium  hydroxide.  Ammonium 
hydroxide  is,  as  a  matter  of  fact,  a  much  weaker  base.  The 
cause  of  greater  or  less  strength  among  the  different  bases 
and  acids  has  not  yet  been  suggested,  but  it  is  to  be  dis- 
cussed in  Chapter  XXV. 


218  ACIDS  AND   BASES;   NEUTRALIZATION 

SUMMARY 

Acids  contain  hydrogen  and  a  radical  consisting  of  a  non-metal  or 
of  non-metallic  elements.  When  the  acid  is  dissolved  in 
water,  tlje  attachment  between  the  hydrogen  and  the  radical 
is  apparently  weakened,  so  that  the  hydrogen  is  displaceable 
by  metals,  and  is  free  enough  to  manifest  its  characteristics 
of  sour  taste,  power  to  redden  litmus,  and  ability  to  combine 
with  the  hydroxyl  of  bases,  thereby  neutralizing  the  latter. 

The  oxides  of  the  non-metallic  elements  form  acids  when  combined 
with  water. 

Bases  contain  the  hydroxyl  group,  OH,  and  a  metallic  element  or 
radical.  When  a  base  is  dissolved  in  water,  the  hydroxyl 
group  becomes  active,  in  the  same  manner  as  the  hydrogen  of 
acids.  It  then  displays  its  characteristics  of  alkaline  taste, 
power  to  turn  litmus  blue,  and  ability  to  neutralize  acids. 

The  oxides  of  the  metallic  elements  form  bases  when  combined  with 
water. 

When  an  acid  and  base  neutralize  each  other,  the  hydrogen  of  the 
one  and  the  hydroxyl  of  the  other  combine  to  form  water  and 
thus  disappear  as  active  components,  while  the  metal  or 
metallic  radical  of  the  base  and  the  non-metallic  radical  of  the 
acid  together  yield  a  salt. 


Questions 

1.  Cream  of  tartar  has  a  sour  taste.     This  taste  is  due  to  the 
presence  of  what  component  in  the  cream  of  tartar  ? 

2.  To  the  presence  of  what  group  of  elements  would  you  attrib- 
ute the  alkaline  taste  of  limewater  (calcium  hydroxide  solution)? 

3.  What  becomes  of  the  components  which  are  responsible  for 
the  sour  taste  and  the  alkaline  taste,  respectively,  when  an  acid 
is  neutralized  by  a  base? 

4.  What  is  a  salt?     What  acid  and  what  base   yield   common 
salt  by  neutralization? 

6.  How  could  you  make  sodium  sulphate  from  (a)  sodium 
hydroxide?  (6)  sodium  chloride?  (c)  sodium  carbonate?  Give 
equations. 


QUESTIONS  219 

6.  If  one  has  spilled  sulphuric  acid  on  his  garment,  why  does 
he  use  ammonia  water  to  prevent  a  hole  being  eaten?     Why  not 
use  sodium  hydroxide? 

7.  Why  does  limewater  correct  sour  stomach? 

8.  What  weight  of  (a)  hydrochloric  acid,   (6)  sulphuric  acid, 
should  be  taken  to  neutralize  40  grams  of  sodium  hydroxide  ? 

9.  What    volume    of    carbon    dioxide    (standard    conditions) 
should  be  passed  into  a  solution  of  40  grams  of  sodium  hydroxide 
to  form  sodium   carbonate  —  2  NaOH  +  C02  ->  Na2C03  +  H20  ? 

10.  Ammonium  salts  are  obtained  from  the  liquors  condensed 
in  gas  works  by  distilling  off  ammonia  and  passing  it  into  acid. 
What  volume  of  ammonia  gas  (standard  conditions)  can  be  caught 
in  one  kilogram  of  sulphuric  acid? 

11.  What  volume  of  potassium  hydroxide  solution  containing 
100  grams  KOH  per  liter  will  just  neutralize  100  c.c.  of  hydrochloric 
acid  containing  200  grams  HC1  per  liter? 

12.  Cite  several  examples  in  which  an  acid  is  made  by  treating 
the  oxide  of  a  non-metal  with  water ;    in  which  a  base  is  made  by 
treating  the  oxide  of  a  metal  with  water. 


CHAPTER  XX 
NOMENCLATURE. 

SEVERAL  acids,  bases,  and  salts  have  been  discussed  in  the 
last  chapter,  and  names  have  been  used  for  them  which  are 
probably  not  as  yet  fully  understood  by  the  pupil.  It  seems 
best,  therefore,  to  pause  at  this  point  and  explain  the  system 
of  naming  which  is  in  use  generally  by  chemists. 

235.  As  has  already  been  explained,  when  a  substance 
consists  of  but  two  elements,  its  name  includes  the  names, 
or  at  least  the  roots  of  the  names,  of  each  element,  the  more 
metallic  element  being  taken  first,  and  the  suffix  ide  is  added 
to  the  name  of  the  second  element.     The  suffix  ide  has  this 
significance;   namely,  it   indicates   a   compound   consisting 
usually  of  only  two  elements  : 

HC1  hydrogen  chloride 

NaCl  sodium  chloride 

H2O  hydrogen  oxide 

Na2O  sodium  oxide 

H2S  hydrogen  sulphide 

Na2S  sodium  sulphide. 

236.  It  frequently  happens  that  two  elements  form  more 
than  one  compound  with  each  other  (see  law  of  Multiple 
Proportions).     One  method  of  distinguishing  the  different 
compounds  in  such  a  series  is  by  using  the  Greek  or  Latin 
prefixes  before  the  name  of  the  non-metal  in  order  to  indicate 
the  number  of  atoms  of  this  element. 

220 


NOMENCLATURE  221 

CO  carbon  monoxide 

CO2  carbon  dioxide 

SOs  sulphur  trioxide 

CCLj  carbon  tetrachloride 

PCls  phosphorus  pentachloride. 

237.  When  a  compound  of  two  elements  is  an  acid  and 
it  is  wished  to  indicate  this  fact,  the  word  acid  is  added  to 
the  name,  and  the  names  of  the  two  elements  are  contracted 
into  a  single  word  which  is  now  used  as  an  adjective  with  the 
common  English  adjective  ending  ic  instead  of  ide. 

HC1     hydrochloric  acid 
H2S      hydrosulphuric  acid 
HBr    hydrobromic  acid. 

On  passing  from  the  simpler  acids  containing  only  two 
elements  to  the  more  complicated  oxy-acids  which  contain 
three  elements,  the  naming  becomes  simpler  instead  of  more 
complicated.  It  must  be  remembered  that  the  oxy-acids 
were  the  first  acids  to  be  discovered ;  indeed,  it  was  thought 
formerly  that  oxygen  was  the  essential  acid-producing  con- 
stituent of  acids.  It  is  only  comparatively  recently  that 
hydrogen  has  become  recognized  as  the  true  acid-producing 
constituent.  Acids  were  thus  originally  named,  in  accordance 
with  this  erroneous  conception,  after  the  non-metal  whose 
oxide  gave  the  acid  when  it  united  with  water.  For  example, 
carbonic  acid  was  the  acid  derived  from  the 'oxide  of  carbon. 
This  nomenclature  has  proved  perfectly  satisfactory  and  has 
been  retained. 

The  suffix  ic- is  the  common  termination  for  the  names  of 
acids.  Some  of  the  non-metals  have  more  than  one  oxide 
and,  corresponding  to  the  different  oxides,  yield  more  than 
one  acid.  In  such  cases  the  suffix  ic  is  retained  for  the 


222 


NOMENCLATURE 


commonest,  or  the  most  important,  of  these  acids.  For  an 
acid  containing  less  oxygen,  the  suffix  ous  is  used  (eras  is 
another  common  English  adjective  ending). 

Thus,  the  most  important  acid  from  sulphur  is  sulphuric 
acid,  derived  from  sulphur  trioxide.  From  sulphur  dioxide 
an  acid  with  less  oxygen  is  obtained  which  is  called  sul- 
phuTous  acid.  The  acid  which  contains  hydrogen  and  sul- 
phur alone  is  distinguished  from  these  oxy-acids  in  that  the 
prefix  hydro  is  used. 


OXIDE 

FORMULA  OF  ACID 

NAME  OP  ACID 

H2S 

Hydro  sulphuric  acid 

S02 

H2S03 

Sulphurous  acid 

S03 

H3S04 

Sulphuric  acid 

P203 

H3P03 

Phosphorous  acid 

P205 

H3P04 

Phosphoric  acid 

238.  Some  elements,  notably  chlorine,  yield  more  than 
two  acids.  In  such  cases  the  prefix  per  is  used  to  denote 
a  greater  oxygen  content  than  that  of  the  plain  ic  acid,  and 
the  prefix  hypo  to  denote  a  lower  oxygen  content  than  that 
of  the  plain  oiis  acid.  To  illustrate  this  point,  the  chlorine 
acids,  including  the  acid  with  no  oxygen,  are  given  in  the 
following  table : 


OXIDE 

FORMULA  OF  ACID 

NAME  OF  ACID 

HC1 

hydro  chloric  acid 

C12O 

HC10 

hypo  chlorous  acid 

C1203 

HC102 

chlorows  acid 

C1205 

HC103 

chloric  acid 

C1207 

HC1O4 

perchloric  acid 

NOMENCLATURE 


223 


239.  Bases  always  consist  of  a  metallic  element  combined 
with  hydroxyl.     In  naming  bases,  the  name  of  the  metal 
is  first  given,  and  this  is  followed  by  the  word  hydroxide. 

NaOH        sodium  hydroxide 

KOH          potassium  hydroxide 

Zn(OH)2     zinc  hydroxide. 

The  termination  ide  usually  signifies  that  a  compound  con- 
taining only  two  elements  is  indicated.  Still  its  use  in  this 
case  is  justified  on  the  ground  that  the  OH  group  behaves 
quite  like  a  single  atom. 

240.  Salts  may  be  derived  by  the  neutralization  of  bases 
and  acids  and  are  named  in  accordance.     Salts  containing 
but  two  elements  are  named  according  to  the  ru!e  already 
given  for  compounds  of  only  two  elements.     Salts  of  oxy- 
acids  are  named  after  the  acids,  except  that  the  ending  ic 
is  changed  to  ate  and  the  ending  ous  is  changed  to  ite.     The 
prefixes  hypo  and  per  are  retained  in  the  names  of  the  oxy- 
salts,  but  the  prefix  hydro  is  not  retained  in  the  names  of 
the  salts  derived  from  the  hydro-acids. 


FORMULA 
OF  ACID 

NAME  OP  ACID 

FORMULA  OF 
SODIUM  SALT 

NAME  OF  SALT 

HC1 

hydrochloric  acid 

NaCl 

sodium  chloride 

HC10 

hypochlorous  acid 

NaCIO 

sodium  hypochlorite 

HC102 

chlorous  acid 

NaC102 

sodium  chlorite 

HC103 

chloric  acid 

NaC103 

sodium  chlorate 

HC104 

perchloric  acid 

.  NaC104 

sodium  perchlorate 

H2S 

hydrosulphuric  acid 

Na2S 

sodium  sulphide 

H2S03 

sulphurous  acid 

Na2S03 

sodium  sulphite 

H2S04 

sulphuric  acid 

Na2S04 

sodium  sulphate 

HN02 

nitrous  acid 

NaNO2 

sodium  nitrite 

HN03 

nitric  acid 

NaNO3 

sodium  nitrate 

B.    AND   W.    CHEM. 15 


224  NOMENCLATURE 

Questions 

1.  Name  the  compound  whose  formula  is  KI. 

2.  Name  the  acid  from  which  KI  is  derived. 

3.  What  acid  yields  iodates  on  neutralization? 

4.  What  acid  yields  per  chlorates? 

6.   Name  the  sodium  salt  of  phosphorous  acid. 

6.  Name  the  potassium  salt  of  hypochlorous  acid. 

7.  What  acid  yields  hypochlorites  ? 

8.  Name  salts  derived  from  silicic  acid. 

9.  Name   compounds  having  the  following  formulas:    NaBr, 
HBr,  KBrO,  Ca(Br03)2,  KI04,  H2S,  K2S,  CaS03,  KN02,  Zn(N03)2. 

10.   Give  the  formulas  of  potassium  chloride,  potassium  sulphide, 
potassium  sulphite,  ammonium  sulphate,  potassium  perchlorate. 


CHAPTER  XXI 

THE  METALS 

IT  has  already  been  seen  that  it  is  impossible  to  deal  with 
the  chemistry  of  the  non-metals  without  describing  the  com- 
pounds that  they  form  with  the  metals.  We  have  already 
acquired  some  knowledge  of  the  metals  sodium  and  calcium 
and  a  fragmentary  knowledge  of  some  of  the  other  metals ; 
to  make  this  knowledge  more  systematic,  we  shall  devote 
this  chapter  to  the  study  of  the  metals  and  the  following 
chapter  to  the  methods  of  winning  the  metals  from  the  rocks 
and  earths  in  which  they  are  found  in  nature. 

241.  Physical  Properties  of  Metals.  A  metal  can  be 
almost  unerringly  recognized  even  at  a  glance  by  its  peculiar 
luster,  —  the  so-called  metallic  luster.  It  is  very  difficult 
to  describe  metallic  luster,  but  it  is  something  that  would 
never  be  confused  with  the  luster  of  non-metallic  substances 
such  as  glass,  or  sulphur  crystals. 

Besides  having  metallic  luster,  all  metals  have  a  high 
degree  of  conductivity  for  heat  and  electricity.1 

Many  of  the  common  metals  are  strong,  or  hard,  or  ductile, 
or  tenaceous,  or  malleable,  or  heavy,  or 'difficult  to  melt, 
but  none  of  these  properties  are  distinctive  of  the  metals, 

1  It  is  true  that  graphite  (carbon)  and  some  of  the  sulphides 
and  oxides  of  the  heavy  metals,  for  example,  magnetic  oxide 
of  iron,  possess  metallic  luster  and  conduct  electricity.  They  thus 
possess  the  distinctive  physical  properties  of  the  uncombined  metals, 
but  this  does  not  impair  the  correctness  of  the  above  statement 
that  all  metals  have  these  properties. 

225 


226  THE   METALS 

as  are  the  metallic  luster  and  the  conductivity  for  heat  and 
electricity.  For  example,  iron  is  strong,  hard,  and  difficult 
to  melt,  but  lead  is  weak,  soft,  and  easily  melted.  Gold 
and  copper  are  ductile  and  malleable,  but  chromium,  man- 
ganese, and  bismuth  are  so  brittle  that  they  can  be  broken 
into  fragments  with  a  hammer.  Platinum  will  barely  melt 
in  the  oxy-hydrogen  blowpipe,  while  sodium  will  melt  below 
the  temperature  of  boiling  water,  and  mercury  melts  at  a 
temperature  40°C.  below  the  melting  point  of  ice.  Gold, 
lead,  and  mercury  are  very  heavy ;  aluminium  and  magnesium 
are  light,  and  sodium  is  so  light,  that  it  floats  on  water. 

To  sum  up  the  physical  characteristics  of  metals:  the 
metallic  luster  and  the  conductivity  for  heat  and  electricity 
are  properties  possessed  by  the  metals  without  exception. 
Numerous  other  properties  are  associated  in  the  popular 
mind  with  metals,  but  they  are  not  invariable  and  distinctive 
properties  of  metals. 

242.  Chemical  Properties  of  Metals.  In  chemical  prop- 
erties the  metals  are  as  different  from  the  non-metals  as  in 
physical  properties.  One  of  the  most  striking  chemical 
characteristics  of  the  metals  has  already  been  mentioned  in 
Chapter  XIX;  namely,  that  the  oxides  of  the  metals  are 
base-forming  in  distinction  to  the  oxides  of  the  non-metals, 
which  are  acid-forming. 

Acids  and  bases  are  opposed  to  each  other  in  their  chemical 
nature,  as  we  may  conclude  from  their  ability  to  neutralize 
each  other.  In  a  similar  way,  the  metals  and  the  non- 
metals  themselves  are  opposed  to  each  other  in  their  chemical 
nature,  for  they  are  capable  of  changing,  or  neutralizing, 
each  other's  properties  when  they  combine.  We  are  familiar, 
for  example,  with  the  striking  properties  of  sodium  and  the 
striking  properties  of  chlorine;  when  these  elements  com- 


HYDROGEN  AS  A  METAL  227 

bine,  these  striking  properties  disappear  and  we  have  ordinary 
common  salt,  which  does  not  have  the  metallic  luster  and 
the  conductance  for  electricity  of  sodium  nor  the  evil  odor 
and  the  yellowish  color  of  chlorine. 

243.  Hydrogen  as  a  Metal.  The  element  hydrogen  does 
not  possess  the  physical  characteristics  of  metals,  but  in 
chemical  properties  it  shows  a  strong  resemblance  to  the 
metals.  It  combines  with  non-metals,  forming  such  com- 
pounds as  hydrogen  chloride  and  water,  but  it  does  not  show 
any  marked  tendency  to  form  compounds  with  the  metals. 
Furthermore,  hydrogen  is  interchangeable  with  the  metals 
in  nearly  all  of  their  compounds.  For  example,  on  treating 
an  acid  such  as  hydrogen  chloride  with  zinc  or  iron,  the 
metals  displace  hydrogen  according  to  the  equations  : 


2HC1  +  Fe-*FeCl2  +  H2. 

Zinc  chloride  and  iron  chloride,  then,  may  be  regarded  as 
hydrochloric  acid  in  which  zinc  or  iron  has  been  substituted 
for  hydrogen.  When  copper  chloride  is  heated  in  hydrogen 
gas,  uncombined  copper  and  hydrogen  chloride  are  obtained  : 


Hydrogen  chloride  may  be  regarded  as  copper  chloride  in 
which  hydrogen  has  been  substituted  for  copper. 

It  is  thus  seen  that  metals,  including  hydrogen,  all  have 
the  same  kind  of  combining  power.  So,  if  one  metal  forms 
a  chloride,  then  all  of  the  metals  including  hydrogen  should 
do  the  same.  It  is  in  fact  true  that  all  of  the  metallic 
elements  form  distinct  chlorides.  Nearly  all  likewise  form 
oxides. 

Not  only  are  metals  interchangeable  with  hydrogen  in 


228  THE  METALS 

the  simple  compounds  hydrogen  chloride  and  water,  but 
also  in  the  oxy-acids  such  as  sulphuric,  phosphoric,  and 
carbonic  acids,  in  which  hydrogen  is  in  combination  with 
radicals  instead  of  with  single  elements.  In  fact,  sulphates, 
phosphates,  and  carbonates  of  all  the  most  active  metals 
are  well  known.  Whole  mountain  ranges,  for  example,  are 
composed  of  calcium  carbonate  and  magnesium  carbonate, 
and  deposits  of  calcium  sulphate  and  calcium  phosphate, 
of  iron  carbonate,  copper  carbonate,  zinc  carbonate,  and 
lead  carbonate  are  often  found  in  the  earth. 

244.  Electrical  Nature  of  Chemical  Attraction.  It  is 
clear  from  what  has  already  been  said  that  it  is  oppositeness 
in  chemical  nature  that  makes  it  possible  for  two  elements 
to  form  compounds.  We  know  that  when  an  electric  current 
is  passed  through  water,  the  latter  is  decomposed  into 
hydrogen  and  oxygen;  likewise  hydrochloric  acid  is  de- 
composed into  hydrogen  and  chlorine.  The  hydrogen  is 
liberated  in  each  case  at  the  negative  electrode,  and  since  it 
is  known  that  unlike  electric  charges  attract  each  other, 
whereas  like  charges  repel,  it  must  be  that  the  hydrogen  in 
the  compound  is  electrically  positive  in  order  to  be  attracted 
to  the  negative  electrode.  According  to  the  same  reasoning, 
since  the  oxygen  and  chlorine  are  liberated  at  the  positive 
electrode,  these  must  be  electrically  negative  as  they  exist 
in  their  compounds.  Like  hydrogen,  the  metals  must  all  be 
electro-positive  in  their  compounds ;  for  they  are  always  set 
free  at  the  negative  electrode  when  the  current  is  passed 
through  solutions  of  any  of  their  salts. 

We  are  certain  that  in  many  compounds  the  attractive 
force  holding  the  elements  together  is  in  some  way  dependent 
on  their  electrical  condition.  It  may  be  that  this  is  true  of 
all  chemical  compounds,  although  this  would  be  too  sweeping 


ALLOYS  229 

a  statement  to  make  unconditionally  in  the  light  of  our 
present  knowledge. 

At  all  events  this  statement  may  be  made  without  reser- 
vation :  that  in  compounds  of  metals  and  non-metals  the 
metals  are  electro-positive  in  nature  and  the  non-metals  are 
electro-negative. 

245.  Compounds  of  Non-Metals  with  Non-Metals.  —  It 
is  true  that  non-metals  often  form  compounds  among  them- 
selves ;  as,  for  example,  in  the  oxides  of  carbon,  sulphur,  and 
phosphorus.     There  must  be  some  difference  in  nature  to 
make  the  chemical  union  possible,  and  if  this  difference  is 
electrical,  it  must  be  that  the  carbon,  sulphur,  and  phos- 
phorus are  forced  to  play  the  electro-positive  role  in  these 
compounds  because  the  oxygen  has  so  much  greater  ability 
to  play  the  electro-negative  role.     It  is  true,  however,  as 
a  general  rule  that  the  compounds  of  the  non-metals  among 
themselves  are  feebler  (that  is,  much  more  easily  broken 
apart)    than   the   compounds  of   the   non-metals  with   the 
metals. 

246.  Compounds  of  Metals  with  Metals ;  Alloys.     Metals 
show  even  less  tendency  to  form  compounds  among  them- 
selves than  do  the  non-metals.     When  different  metals  are 
melted  and  poured  together,  alloys  are  obtained :    brass  is 
an  alloy  of  copper  and  zinc ;   bronze  an  alloy  of  copper  and 
tin ;  solder  an  alloy  of  tin  and  lead.     These  alloys  are  not  def- 
inite compounds  of  the  metals,  but  are  in  the  main  mixtures, 
much  like  the  mixture  that  we  obtain  by  pouring  alcohol 
and  water  together.     The  molten  metals  mix,  and  when  the 
mixture  cools,  it  solidifies  to  a  metallic  mass  which  has 
metallic  luster  and  conducts  electricity  as  do  both  of  its 
component  metals. 

The  most  recent  study  of  alloys  has  indeed  proved  that 


230  THE   METALS 

they  sometimes  contain  minute  crystals  of  certain  definite 
compounds  embedded  in  the  mass  of  the  metal.  These 
crystals  can  sometimes  be  observed  with  a  microscope  on 
a  polished  surface  of  the  alloy  that  has  been  carefully  etched 
with  acid.  At  best,  however,  the  power  of  metals  to  combine 
chemically  with  other  metals  is  very  limited. 

247.  Classification  of  the  Metals.      Now  that    we  have 
shown  in  a  general  way  how  metals  are  different  from  non- 
metals,  let  us  divide  them  into  groups  or  classes  according 
to  their  properties.     They  might  be  classed  according  to 
their  base-forming  properties  or  according  to  their  behavior 
with  acids,  or  according  to  their  physical  properties.     We 
shall  group  them  into  three  main  classes ;  namely,  the  alkali 
metals,  the  earth-forming  metals,   and  the  heavy  metals. 
This  grouping  is  in  the  main  based  on  the  physical  property 
of  weight,  although  the  chemical  properties  also  follow  this 
grouping  to  a  great  extent. 

248.  The  alkali  metals  are  those  from  which  the  caustic 
alkalies  are  obtained.     We  have  already  considered  the  two 
of  most  frequent  occurrence  ;  namely,  sodium  and  potassium, 
which   yield   the  bases,  sodium   hydroxide   and   potassium 
hydroxide  respectively.     These  bases  are  extremely  soluble 
and  their  concentrated  solutions  are  caustic  or  corrosive  to 
the  flesh  or  to  any  animal  or  vegetable  tissues.     The  terms 
alkali  and  base  are  somewhat  interchangeable.     A  base,  as 
seen  in  Chapter  XIX,  is  a  substance  that  contains  an  hydroxyl 
group  and  can  neutralize  an  acid.     An  alkali  is  a  substance 
that  dissolves  freely  in  water  and  then  shows  to  a  marked 
degree  the  characteristics  of  soluble  bases.     For  example, 
an  alkali  turns  red  litmus  blue,  it  has  an  alkaline  taste,  it  is 
useful  in  cleansing  or  removing  grease  and  dirt. 

As  already  seen,  metallic  sodium  and  potassium  are  light 


THE  ALKALI   METALS  231 

in  weight,  —  they  will  float  on  water  ;  they  are  soft  and  can 
be  cut  with  a  knife  almost  as  easily  as  cheese;  they  melt 
below  the  boiling  point  of  water.  These  metals  conduct 
electricity  and  show  a  bright  metallic  luster  on  freshly 
exposed  surfaces.  Exposed  to  the  air,  the  surfaces  soon 
grow  dull  on  account  of  the  formation  of  a  layer  of  oxide 
or  hydroxide  on  the  surface. 

Sodium  and  potassium  both  react  violently  when  thrown 
on  cold  water.  They  float  around,  giving  a  rapid  evolution 
of  hydrogen,  and,  especially  with  potassium,  the  heat  of  the 
reaction  is  so  intense  as  to  set  fire  to  the  hydrogen.  The 
reaction  yields  the  hydroxides  : 


K    +H20->KOH    +  H. 

Sodium  and  potassium  combine  violently  with  oxygen  and 
chlorine  when  heated  in  these  gases.  The  alkali  metals  are, 
in  fact,  the  most  active  of  the  metallic  elements.  The 
oxides  react  energetically  with  water  and  give  the  hydroxides. 
For  example,  sodium  oxide  reacts  as  follows  : 

Na20  +  H20-*2NaOH. 

It  has  already  been  mentioned  that  when  sodium  burns 
in  a  plentiful  supply  of  air,  a  higher  oxide  —  a  so-called 
peroxide  —  is  formed.  Potassium  also  forms  a  peroxide, 
K2O4.  When  either  of  these  peroxides  is  treated  with  water, 
the  ordinary  hydroxide  is  obtained  just  as  from  the  mon- 
oxide, and  the  excess  of  oxygen  escapes  as  oxygen  gas  : 

2  K204  +  2  H20-M  KOH  +  3  O2. 

Since  the  alkali  metals  are  the  most  energetic  of  the 
metallic  elements  and  show  so  great  a  tendency  to  react 


232  THE   METALS 

with  the  abundant  substances,  water  and  oxygen,  it  is  not 
strange  that  these  metals  are  never  found  uncombined  in 
nature.  Nearly  all  the  salts  of  the  alkali  metals  are  very 
soluble  in  water. 

The  most  generally  useful  and  important  compounds  of 
the  alkali  metals  are  the  hydroxides  and  the  carbonates. 
The  hydroxides  constitute  the  caustic  alkalies,  the  carbonates 
are  mild  alkalies.  We  might  expect  that  the  carbonates 
would  be  neutral  and  not  alkaline,  because  they  are  salts 
and  may  be  obtained  by  neutralizing  a  base  with  an  acid : 

2  NaOH  +'  H2CO3  ->  2  H2O  +  Na2CO3. 

But  it  must  not  be  lost  sight  of  that  sodium  hydroxide  is  an 
extremely  strong  base,  whereas  carbonic  acid  is  a  very  weak 
acid.  This  fact  permits  the  neutralization  to  reverse  itself 
to  a  certain  extent  when  water  has  access  to  salts  of  this 
type  and  hence  a  little  of  the  base  is  present  in  the  solution 
(see  Hydrolysis,  p.  405). 

249.  Ammonium.     The    ammonium    group,    or    radical 
(NH4),  as  we  have  already  seen  in  Chapter  XIX,  acts  chemi- 
cally very  much  like  a  metal.     Its  compounds  are  almost 
without  exception  soluble,  as  are  those  of  the  alkali  metals, 
and  like  them,  its  hydroxide  is  very  soluble  in  water  and 
is  a  pronounced  base  (or  alkali).     Hence  the  hypothetical 
metal  ammonium  is  classed  among  the  alkali  metals. 

250.  The    Earth-forming    Metals.     Our    next    class    of 
metals,  the  earth-forming  metals,  is,  as  the  name  implies, 
a  group  whose  compounds  form  a  large  part  of  the  earthy 
matter  of  which  the  surface  of  our  earth  is  made.     The 
most  important  metals  in  this  class  are  magnesium,  calcium, 
barium,  and  aluminium. 

In  physical  properties  these  metals  are  much  harder  and 


THE   EARTH-FORMING  METALS  233 

more  tenacious  than  the  alkali  metals,  although  they  fall 
a  good  deal  below  iron  in  these  respects.  They  are  light  in 
weight  as  compared  with  the  heavy  metals  iron,  copper,  and 
zinc,  but  they  are  heavier  than  the  alkali  metals.  One  of 
the  very  valuable  properties  of  aluminium  is  its  lightness, 
for  it  approaches  iron  in  strength  and  yet  it  is  of  far  less 
weight.  These  metals  all  have  fairly  high  melting  points. 

The  earth-forming  metals  are  like  the  alkali  metals  in 
that  they  could  by  no  possibility  remain  long  uncombined 
in  the  earth,  and  as  we  should  expect,  they  are  always  found 
in  a  state  of  combination. 

Of  the  metals  mentioned  in  this  class  calcium  and  barium 
are  most  like  the  alkali  metals,  for  they  react  with  cold  water 
and  liberate  hydrogen  : 

Ca  +  H2O->Ca(OH)2  +  H2, 
Ba  -f  H2O->Ba(OH)2  +  H2. 

Likewise,  their  oxides  react  energetically  with  water  to  form 
hydroxides  : 

H20->Ca(OH)2, 

H2O-^Ba(OH)2. 


The  hydroxides  are  not  so  soluble  as  the  hydroxides  of  the 
alkali  metals  and  are  not  so  strongly  basic.  Calcium  and 
barium  are  often  called  alkaline  earth  metals  to  show  that 
they  are  intermediate  between  alkali  metals  and  the  more 
strictly  earth  metals. 

Magnesium  and  aluminium  do  not  react  appreciably 
with  cold  water.  Their  hydroxides  are  bases  and  react 
with  acids  to  form  salts;  magnesium  hydroxide  is  almost 
insoluble,  and  aluminium  hydroxide  is  still  less  soluble  and 
withal  a  very  weak  base.  Although  these  bases  are  insoluble 


234  THE   METALS 

in  water,  they  dissolve  easily  in  dilute  acid  solutions  because 
their  salts,  which  are  soluble,  are  formed. 

All  of  the  earth-forming  metals  react  vigorously  with  acids 
with  displacement  of  hydrogen  and  formation  of  the  cor- 
responding salt: 

2  Al  +  6  HC1  ->2  A1C13  +  3  H2, 
Mg  +  H2S04-*MgS04  +  H2. 

These  metals  are  thus  shown  to  be  more  active  than  hydrogen, 
as  is  also  shown  by  the  fact  that  it  is  impossible  to  reduce 
their  oxides  by  heating  them  in  hydrogen. 

Magnesium  and  barium,  like  calcium,  are  found  frequently 
in  nature  as  the  carbonates. 

Aluminium  is  the  most  abundant  of  the  metallic  elements 
in  the  earth's  crust.  It  occurs  as  silicate  in  clay  and  in  most 
of  the  rocks  of  the  earth's  crust.  Metallic  aluminium  is 
obtained  almost  solely  by  the  electrolysis  of  the  hydrated 
oxide  which  is  found  in  the  earth  in  fair  abundance  as  the 
mineral  bauxite. 

251.  The  Heavy  Metals.  The  remaining  metals  that 
we  have  to  consider  we  shall  group  together  under  the  title 
of  the  heavy  metals.  The  most  important  ones  of  this 
group  are  copper,  silver,  gold,  mercury,  platinum,  zinc,  tin, 
lead,  chromium,  manganese,  iron,  and  nickel,  and  it  may  be 
seen  that  this  list  includes  all,  except  aluminium,  of  the 
metals  that  are  familiar  in  our  everyday  life. 

As  the  name  implies,  these  metals  are  all  of  high  specific 
gravity.  They  possess  to  a  high  degree  the  characteristic 
physical  properties  of  metals ;  that  is,  metallic  luster  and  con- 
ductivity for  heat  and  electricity.  Most  of  them  have  great 
strength  and  considerable  hardness,  which  make  them  useful 
for  structural  purposes  and  for  making  machines  and  utensils. 


THE   HEAVY  METALS  235 

In  chemical  properties  the  metallic  character  is  not  so 
strongly  marked  in  this  group  as  in  the  other  two  groups. 
The  heavy  metals  form  hydroxides  which  are  capable  of 
neutralizing  acids  and  forming  salts,  for  example : 

Zn(OH)2  +  H2S04-*2  H2O  +  ZnSO4. 

But  the  hydroxides  are  hardly  to  be  called  alkalies ;  they  are 
only  very  weak  bases.  Most  of  these  bases  are  very  insoluble 
in  water,  and  only  one  or  two  of  them  are  strong  enough  to 
barely  impart  to  litmus  the  blue  color  that  is  the  characteristic 
effect  of  bases. 

As  a  class,  the  heavy  metals  are  far  less  active  chemically 
than  the  alkali  or  the  earth-forming  metals.     They  do  not 
react  to  any  great  extent  with  dry  air  or  with  pure  water, 
and  this  freedom  from  attack  is  one  of  the 
properties  that  makes  them  useful  metals.  Manganese 

252.   Varying  Activity  of  the  Heavy  Metals.       chromium 
The  heavy  metals  may  be  graded  according       iron 
to  their  chemical  activity,  and  in  this  respect       Nickel 
they  fall  in  the  order  indicated  in  the  accom-       Tin 
panying  list. 

All  the  metals  standing  above  hydrogen  in 
this  list  are  more  active  than  hydrogen  and       Mercury 
can  displace  that  element  from  acids.     Zinc       Silver 
acts  vigorously  with  dilute  sulphuric  or  hydro-       Platinum 
chloric  acid,   and   hydrogen   gas   is  evolved.       Gold 
Iron  acts  somewhat  less  vigorously.     Tin  and  Relative  Activ- 

'  ity  of  Metals 

lead    displace    hydrogen,    only    very    slowly. 
Metals  below  hydrogen  are  less  active  and  will  not  displace 
hydrogen  at  all  from  acids.     For  example,  if  a  clean  piece 
of  copper  is  placed  in  dilute  sulphuric  or  hydrochloric  acid, 
no  reaction  occurs.     (It  is  true  that  copper  reacts  freely 


236  THE   METALS 

with  nitric  acid  and  a  gas  is  given  off,  —  but  this  gas  is  not 
hydrogen.) 

The  metals  higher  in  the  list  are  not  only  more  active  in 
displacing  hydrogen,  but  they  are  also  more  active  in  com- 
bining with  non-metals ;  for  example,  the  oxygen  of  the  air. 
Bright  polished  surfaces  of  iron,  zinc,  and  lead  do  not  remain 
bright  long  when  exposed  to  the  air,  particularly  if  the  air  is 
moist.  Copper  also  tarnishes  when  exposed  to  the  weather. 
When  the  film  of  oxide  first  formed  on  the  surface  of  a  metal 
is  impervious,  further  action  of  the  weather  is  stopped; 
whereas  if  the  film  is  porous,  as  is  the  case  with  iron  rust, 
the  corrosion  may  continue  until  the  metal  is  all  eaten 
away. 

Silver,  platinum,  and  gold,  which  are  known  as  precious 
metals  on  account  of  their  value,  are  so  inactive  that  they 
completely  withstand  the  action  of  the  weather. 

Both  copper  and  silver  coins  darken  when  carried  in  the 
pocket,  and  silver  spoons  grow  black  when  they  are  used  in 
eating  eggs.  This  blackening  is  caused  by  sulphur  com- 
pounds which  are  present  in  small  amounts  in  the  perspira- 
tion and  in  eggs.  Copper  and  silver  show  a  peculiarly  strong 
inclination  to  form  compounds  with  sulphur,  and  these  sul- 
phides are  very  black,  so  that  they  are  very  conspicuous 
on  the  metallic  surface. 

Platinum  and  gold  are  almost  perfectly  resistant  to  cor- 
rosion or  to  attack  of  any  kind.  Not  only  are  they  un- 
attacked  by  the  ordinary  acids,  but  they  are  not  even  acted 
on  by  the  oxidizing  acid,  nitric  acid,  which  reacts  readily 
with  copper,  silver,  and  mercury.  As  was  seen  under  the 
topic  "  aqua  regia,"  gold  and  platinum  are,  however,  attacked 
by  a  mixture  of  hydrochloric  and  nitric  acids.  Gold  and 
platinum  chlorides  are  formed  by  the  action,  and  these  com- 


SUMMARY  237 

pounds  being  soluble,  the  metals  appear  to  dissolve  in  the 
aqua  regia. 

253.  Occurrence  in  Nature.  According  to  their  activity, 
we  can  predict  with  some  certainty  which  of  the  heavy  metals 
will  be  found  combined  and  which  uncombined  in  the  earth. 
The  metals  above  hydrogen  are  not  found  uncombined, 
except  that  metallic  iron  is  sometimes  found  in  meteorites. 
These  metals  are  often  found  in  large  deposits  as  oxides,  sul- 
phides, and  carbonates. 

The  metals  below  hydrogen  are  all  found  at  times  un- 
combined in  nature.  Copper  is  more  frequently  found  as 
the  oxide  or  sulphide.  Silver  and  mercury  are  found  often 
as  sulphide  and  often  in  the  uncombined  state.  Gold  and 
platinum  are  usually  found  in  the  uncombined  state. 

SUMMARY 

The  metals  form  a  class  of  elements  as  distinguished  from  the  non- 
metals. 

The  principal  physical  characteristics  by  which  metals  differ  from 
non-metals  are  that  they  possess  metallic  luster  and  con- 
ductivity for  heat  and  electricity. 

Chemically,  metals  are  base  formers  and  non-metals  are  acid 
formers. 

Classes  of  metals:  We  have  grouped  the  metals  into  three  main 
classes ;  namely,  the  alkali  metals,  the  earth-forming  metals, 
and  the  heavy  metals. 

The  alkali  metals  are  extremely  active  chemically  and  their  hydrox- 
ides are  very  strong  bases,  the  so-called  caustic  alkalies. 

The  earth-forming  metals  are  also  very  active,  but  they  do  not 
furnish  such  strong  bases.  Many  of  their  compounds  are 
insoluble  in  water  and  earthy  in  their  nature ;  a  large  part 
of  the  earth's  crust  is  composed  of  these  compounds. 

The  heavy  metals  are  heavy  in  weight  and  inactive  chemically  in 
comparison  with  the  other  two  classes.  The  heavy  metals 
may  be  divided  into  two  groups  according  to  their  activity : 


238  THE   METALS 

those  standing  above  hydrogen  in  this  respect  and  those 
standing  below.  The  former  are  found  combined,  the  latter 
are  frequently  found  free  in  nature. 

Questions 

1.  How  can  you  tell  a  metal  from  a  non-metal? 

2.  Why  is  hydrogen  regarded  as  a  metal  in  its  chemical  relations  ? 

3.  Can  metals  unite  chemically  with  metals  ?     What  are  alloys  ? 

4.  Can  non-metals  unite  with  non-metals?     Give  illustrations. 
6.  What  was  the  basis  of  the  classification  of  the  metals  into 

three  groups  in  this  chapter? 

6.  What  are  the  principal  chemical  characteristics  of  the  alkali 
metals  ? 

7.  What  hypothetical  metal  is  classed  with  the  alkali  metals? 

8.  How  do  the  metals  of  the  earth-forming  group  differ  from 
those  of  the  alkali  group  in  general  physical  properties? 

9.  Into  what  group  would  the  precious  metals  naturally  fall? 

10.  Which  of  the  metals  would  you  expect  to  find  uncombined 
in  nature  ? 

11.  Which  class  of  metals  can  best  be  used  for  structural  work? 
Why? 

12.  Which  of  the  heavy  metals  can  best  be  used  for  jewelry? 

13.  Why  is  zinc,  which  is  a  more  active  metal  than  iron,  superior 
to  iron  for  use  on  roofs  ? 

14.  Why  is  the  sheet  iron  used  in  the  canning  industry  always 
coated  with  tin?     (Food  products  often  contain  weak  acids.)     Is 
tin  a  perfect  metal  for  the  purpose  ? 

15.  How  is  the  iron  of  bridges  and  other  structures  guarded  from 
corrosion? 


CHAPTER  XXII 

METALLURGY 

254.  Minerals  and  Ores.  It  has  been  seen  in  the  preced- 
ing chapter  that  the  metallic  elements,  except  sometimes 
copper,  mercury,  and  the  precious  metals,  —  silver,  platinum, 
and  gold,  —  are  found  in  nature  combined  with  non-metals. 
Such  compounds,  and  indeed  all  pure  substances  occurring 
in  rocks,  are  known  as  minerals. 

Sometimes  large  deposits  of  pure  mineral  are  found,  but 
more  often  the  valuable  mineral  is  found  in  veins  and  pockets 
in  other  rock,  or  disseminated  through  it,  and  it  is  impossible 
to  treat  the  mineral  in  the  metallurgical  process  without 
treating  also  large  amounts  of  worthless  rock. 

Such  a  mixture  of  rock  and  mineral  or  native  metal  is 
called  an  ore  when  the  cost  of  mining  and  extracting  the 
metal  from  it  is  less  than  the  value  of  the  metal  obtained. 

Thus  a  rock  which  contains  as  little  as  a  gram  of  gold  per 
ton  may  be  considered  an  ore,  if  it  occurs  in  a  large  deposit 
and  in  a  locality  where  it  can  be  cheaply  handled.  Few  rocks 
contain  so  small  an  amount  of  iron  as  this  and  many  of  the 
common  rocks  contain  several  per  cent  of  iron ;  to  be  of  use 
as  an  iron  ore  the  rock  must  consist  almost  exclusively  of 
the  iron  mineral. 

In  the  table  on  the  next  page  are  given  the  names  and 
chemical  formulas  of  the  most  important  minerals  of  the 
commoner  metals. 

B.  AND  W.  CHEM. 16  239 


240 


METALLURGY 


METAL 

MINERAL 

FORMULA  OF  MINE«AL 

Iron 

Hematite 
Magnetite 
Siderite 
Limonite 

Fe203 
Fe304 
FeC03 
Fe203  •  3  H20 

Copper 

Native  Copper 
Chalcopyrite 
Cuprite 
Chalcocite 

Cu 
CuFeS2 
Cu20 
Cu2S 

Lead 

Galena 

PbS 

Tin 

Cassiterite  or  Tinstone 

Sn02 

Zinc 

Zinc  Blende 
Zincite 
Calamine 

ZnS 
ZnO 
Zn2SiO4 

Mercury 

Cinnabar 

HgS 

Silver 

Native  Silver 
Argentite  or  Silver  Glance 
Cerargyrite  or  Horn  Silver 
Proustite  or  Ruby  Silver 

Ag 
Ag2S 
AgCl 
3  Ag2S  •  As2S3 

Gold 

Native  Gold 
Calaverite 

Au 
AuTe2 

Platinum 

Native  Platinum 

Pt 

Aluminium 

Bauxite 

Al20a  •  2  H20 

Nickel 

Millerite 

NiS 

Antimony 

Stibnite 

Sb2S3 

IRON 

255.  Iron  is  the  most  important  of  all  the  metals.  Its 
cheap  production  and  the  development  of  different  grades 
such  as  cast  iron,  wrought  iron,  and  steel,  varying  in  hard- 


IRON 


241 


ness  and  strength,  have  made  possible  the  enormous  growth 
of  railroads,  the  development  of  building  construction,  and 
the  expansion  of  industrial  operations  within  recent  years. 

Although  iron  is  found  abundantly  in  nature  as  sulphide 
and  silicate  and  to  some  extent  as  carbonate,  only  a  very 
small  amount   of  the 
iron  produced   comes 
from    these     sources. 
The  principal  ores  are 
the  oxides,  which  fre- 
quently occur  in   ex- 
tensive   deposits    and 
can  be  mined  cheaply 
on  a  large  scale.     The 
metallurgical       treat- 
ment    of    the    oxide 
ores  is  also  compara- 
tively simple,  for  the 
oxygen  is  readily  re- 
moved when  the  ore  is  heated  with  some  form  of  carbon. 
Charcoal  is  used  in  a  few  places  for  this  purpose,  but  coke 
is  more  commonly  used.     To  make  iron  as  cheaply  as  is 
to-day  demanded,  operations  must  be  carried  out  on  a  large 
scale  so  that  all  possible  economies  may  be  introduced. 

256.  Blast  Furnace.  The  reduction  of  the  iron  ore  is 
carried  out  in  a  blast  furnace  (Figure  42).  Blast  furnaces 
are  often  built  as  high  as  100  feet;  they  consist  of  a  steel 
shell  lined  with  fire  brick.  Since  the  ore,  the  coke,  and  other 
materials  which  must  sometimes  be  mixed  with  the  ore  are 
put  into  the  furnace  from  the  top,  it  is  necessary  to  provide 
some  form  of  hoisting  apparatus. 

The  rock  which  is  associated  with  the  iron  oxide  in  the  ore 


FIG.  41.  — Hematite. 


242 


METALLURGY 


4ffllL.ro/fc*. 

&.../ron  Ore. 
O....Ltme  Stone. 
4 

fie/  ted  /r  on. 
.....  Drops  of 


D.  .  ..Lower  ftopjoer. 
E..  -tower  Be//. 


.  42.  —  Blast  Furnace. 


BLAST  FURNACE  243 

does  not  melt  easily,  and  it  is  necessary  to  add  something 
which  will  unite  with  it  and  form  more  fusible  compounds. 
(Notice  that  the  blast  furnace  has  no  grate  at  the  bottom  for 
shaking  out  the  ashes,  —  only  two  holes  through  which  liquids 
may  be  withdrawn.)  Anything  which  will  thus  form  a  fusible 
mixture  with  the  worthless  rock  is  called  a  flux.  The  rock 
is  usually  composed  mostly  of  silicon  dioxide  with  consider- 
able aluminium  oxide,  and  other  oxides  are  united  with  part 
of  the  silicon  dioxide  in  the  form  of  silicates.  Aluminium 
silicate  forms  the  principal  part  of  ordinary  clay.  The  silicon 
dioxide  which  is  not  combined  with  the  other  oxides  is  nothing 
but  common  quartz.  This  is  an  acidic  oxide  and,  as  we 
would  suppose  from  our  study  of  acids  and  bases,  a  proper 
flux  for  it  is  a  basic  oxide.  Limestone  is  always  used  for 
this  purpose  in  the  iron  blast  furnace.  The  calcium  carbon- 
ate, of  which  it  is  composed,  is  changed  by  the  heat  into 
calcium  oxide  (lime),  which  unites  with  the  quartz  and  other 
oxides  in  the  charge  and  forms  a  fusible  silicate.  When  the 
latter  flows  from  the  furnace  and  cools  slowly  in  the  air,  it 
looks  much  like  pumice  stone ;  when  it  cools  rapidly,  it  looks 
like  glass.  Indeed  it  is  not  unlike  glass  in  character.  It  is 
known  as  the  slag. 

The  coke  serves  a  double  purpose,  first  to  reduce  the  iron 
oxide,  and  second  to  act  as  a  fuel  and  melt  both  the  reduced 
iron  and  the  slag.  A  small  part  of  the  carbon  of  the  coke  is 
dissolved  in  the  molten  iron.  Heated  air  is  blown  under  a 
high  pressure  through  the  pipes,  known  as  the  tuyeres, 
entering  near  the  base  of  the  furnace.  The  coke  meeting 
this  air  blast  is  burned  to  carbon  dioxide,  but  this  gas 
passes  up  through  the  heated  coke  and  is  reduced  to  carbon 
monoxide, 

CO2+C->2CO. 


244  METALLURGY 

The  carbon  monoxide  passing  through  the  hot  iron  oxide 
in  the  middle  part  of  the  furnace  reduces  it, 

Fe2O3  +  3  CO  ±£  2  Fe  +  3  CO2. 

This  reaction,  however,  is  a  reversible  one,  as  indicated  by 
the  double  arrow,  and  it  only  proceeds  in  the  desired  direc- 
tion towards  the  right  so  long  as  an  abundance  of  carbon 
monoxide  compared  with  the  amount  of  carbon  dioxide  is 
present.  Consequently,  the  escaping  gases  have  a  large  con- 
tent of  carbon  monoxide;  the  amount  usually  exceeds  20 
per  cent  by  volume  of  the  total  gas  and  it  possesses  about 
one  half  the  heating  value  of  the  coke  used  in  the  furnace. 
The  blast  furnace  gas  is  burned  outside  of  the  furnace  to 
heat  boilers,  and  to  preheat  the  air  blown  into  the  furnace. 
It  is  also  used  in  gas  explosion  engines  to  furnish  power. 

The  ore  and  limestone  are  mixed  and  fed  with  alternate 
layers  of  coke  into  the  top  of  the  furnace.  The  charge 
gradually  sinks  through  the  shaft  and  undergoes  various 
changes  as  it  approaches  the  point  where  the  air  enters, 
which  is  the  hottest  part  of  the  furnace.  In  the  upper  part 
it  is  merely  heated  and  dried ;  in  the  central  part  most  of  the 
reduction  takes  place,  but  the  heat  is  not  intense  enough  to 
melt  the  iron.  As  the  charge  descends  into  the  narrower 
part  of  the  furnace  it  becomes  very  hot  and  the  iron  alloys 
with  some  of  the  carbon.  Now  pure  iron  melts  only  at  1520° 
C.,  a  temperature  which  is  hardly  reached  in  any  part  of  the 
furnace ;  but  iron  alloyed  with  a  few  per  cent  of  carbon  melts 
as  low  as  1125°  C.  The  molten  iron  sinks  into  the  crucible 
at  the  bottom  of  the  furnace.  The  flux  arid  foreign  rock 
melt  together  as  slag,  which  floats  on  the  molten  iron  and 
protects  it  from  the  oxidizing  influence  of  the  blast. 

At  the  bottom  of  the  crucible  is  the  metal  tap  through 


BESSEMER  PROCESS  245 

which  the  molten  iron  is  withdrawn  about  every  six  hours. 
At  other  times  the  tap  is  kept  closed.  Above  this  is  the  slag 
tap  through  which  the  molten  slag  is  withdrawn  every  two 
hours.  The  iron  may  be  either  cast  into  molds,  forming 
what  is  called  pig  iron,  or  it  may  be  converted  into  steel  as 
described  below.  The  slag  is  sometimes  thrown  away  and 
sometimes  used  for  making  cement. 

257.  Cast  Iron.     The  direct  product  of  the  blast  furnace  is 
not  pure  iron.     It  contains  3  to  4  per  cent  of  carbon,  some- 
times as  much  as  3  per  cent  of  silicon  —  from  a  reduction  of 
some  of  the  silicon  dioxide  —  and  manganese,  phosphorus,  and 
sulphur,  if  the  ore  contained  these  elements.     This  impure 
iron  melts  easily  and  is  useful  in  making  castings,  but  it  is 
brittle  and  lacks  strength. 

Nevertheless  it  is  able  to  resist  crushing  and  it  is  largely 
used  in  making  supporting  columns  to  sustain  great  weights 
in  buildings.  It  is  also  used  for  engine  bases  and  supporting 
structures  for  heavy  machinery.  Since  it  expands  at  the 
moment  of  solidifying,  it  is  forced  into  every  corner  of  the 
mold  and  very  sharply  defined  castings  are  thus  obtained. 
On  account  of  its  cheapness  and  the  facility  with  which  it 
is  made  into  castings  enormous  quantities  are  used  where 
brittleness  is  not  too  objectionable  a  feature. 

MANUFACTURE  OF  STEEL 

The  carbon  in  excess  of  2  per  cent  together  with  the 
silicon  and  some  of  the  other  impurities  in  pig  iron  make  it 
brittle  and  weak  and  unsuitable  for  many  purposes.  These 
impurities,  which  are  due  to  the  reducing  action  in  the  blast 
furnace,  can  be  removed  by  an  oxidizing  process. 

258.  Bessemer  Process.     This  method,  which  takes  its 
name  from  the  inventor,  is  carried  out  in  the  so-called  Besse- 


246 


METALLURGY 


mer  converter,  Figure  43.  The  converter,  which  may  hold 
as  much  as  20  tons  of  metal,  is  a  steel  shell  lined  with  silicious 
material;  but  when  the  metal  contains  much  phosphorus 
or  sulphur  it  is  necessary  to  make  the  lining  of  burnt  dolomite 
(limestone  containing  magnesium  carbonate  in  addition  to 
calcium  carbonate).  When  the  silicious  lining  is  used,  the 

method  is  called  the 
acid  process;  and 
when  the  dolomite 
lining  is  used,  the 
method  is  called  the 
basic  process. 

The  converter  is 
partly  filled  with 
molten  iron  from  the 
blast  furnace,  and 
air  under  high  pres- 
sure is  forced  into  the 
hollow  place  in  the 
bottom,  from  which  it 
passes  through  small 
perforations  into  the 
molten  metal.  The 

impurities  are  more  easily  oxidized  by  the  air  than  is  the  iron. 
The  silicon  and  manganese  oxides  formed  (and  magnesium 
phosphate  in  the  basic  process)  float  as  molten  slag  on  the 
surface  of  the  metal,  and  the  carbon  passes  off  as  monoxide 
gas.  The  heat  produced  in  burning  these  impurities  is 
sufficient  to  keep  the  charge  in  a  fluid  condition.  The 
disappearance  of  the  carbon  flame  indicates  that  the  oxida- 
tion is  completed.  It  is  impossible  to  stop  the  process  when 
the  carbon  is  reduced  to  just  the  desired  per  cent,  so  it  is 


FIG.  43. — Bessemer  Converter. 


OPEN   HEARTH  PROCESS 


247 


customary  to  burn  oft  all  the  carbon  and  then  add  the  nec- 
essary amount  in  the  form  of  an  alloy  of  iron,  manganese, 
and  carbon.  The  manganese  improves  the  quality  of  the 
product. 

After  the  iron  has  been  recarburized,  the  converter  is  tipped, 
and  the  contents,  which  are  now  in  the  form  of  steel,  are 
poured  into  large  ladles  from  which  the  ingot  molds  are  filled. 

259.  Open  Hearth  Process.  This  is  carried  on  in  a  fur- 
nace such  as  is  shown  in  Figure  44.  Some  of  these  furnaces 
will  hold  75  tons  or  more  of  charge.  The  hearths  are  lined 


FIG.  44. —  Open  Hearth  Furnace.  Gas  (A)  and  air  (B)  after  passing  over  hot 
bricks  enter  the  furnace  together.  The  iron  (C)  rests  upon  the  hearth  lin- 
ing (D).  The  hot  gaseous  products  heat  the  bricks  in  E  and  F  which  are 
later  made  the  inlets  in  order  to  preheat  the  entering  gas  and  air. 

with  the  same  materials  used  for  converter  linings.  They 
are  heated  by  burning  gas  above  the  charge  ;  both  the  gas  and 
the  air  which  is  necessary  to  burn  it  are  previously  heated  by 
passing  over  hot  bricks  below  the  furnace,  thus  increasing 
the  furnace  temperature. 


248 


METALLURGY 


The  material  used  for  making  steel  in  these  furnaces  is  a 
mixture  of  pig  iron,  scrap  iron  or  steel,  and  oxide  iron  ore. 


FIG.  45.  —  Open  Hearth  Furnaces.     Pouring  Ingots. 

The  scrap  dilutes  the  impurities  in  the  pig  iron,  but  most 
of  the  impurities  are  removed  by  oxidation,  due  partly  to 
the  oxygen  in  th»  air  and  partly  to  the  oxygen  of  the  iron 
oxide.  When  a  sample  shows  that  the  impurities  are  suffi- 


SPECIAL  STEELS  249 

ciently  removed,  the  charge  is  withdrawn  into  ladles,  car- 
burized  if  necessary  in  the  same  manner  as  in  the  Besse- 
mer process,  and  then  cast  into  ingots.  Since  it  is  impossible 
to  take  samples  during  the  treatment  in  the  Bessemer  con- 
verter, the  process  cannot  be  so  well  regulated  as  the  open 
hearth  process,  where  frequent  samples  may  be  taken  and  a 
definite  product  obtained.  For  this  reason,  open  hearth 
steel  is  usually  considered  more  reliable  than  Bessemer 
steel. 

260.  Uses  of  Mild  Steel.     Steel  containing  less  than  0.3 
per  cent  of  carbon  is  known  as  mild  steel.     Most  of  the  steel 
produced  by  the  Bessemer  and  open  hearth  processes  is  of 
this  kind,  and  it  is  used  in  tremendous  quantities  for  railroad 
rails,  for  structural  work  in  buildings  and  bridges,  for  wire 
fences,  for  telegraph  wires,  and  for  rolling  into  sheet  metal. 
The  sheet  metal  may  be  used  uncoated  for  many  purposes. 
For  so  called  tin  cans  it  is  coated  with  tin  to  protect  it  from 
oxidation  and  from  the  action  of  the  food  products  which  may 
be  put  up  in  the  cans.     Sheet  iron  or  steel  is  coated  with  zinc 
(galvanized),  especially  when  it  must  resist  the  corrosive  ac- 
tion of  a  damp  atmosphere.     Sheet  iron  for  house  furnaces  in 
damp  cellars  and  for  roofing  purposes  is  always  galvanized. 

Great  strength  and  toughness  combined  with  a  consider- 
able degree  of  hardness  characterize  mild  steel  and  give  it  its 
great  usefulness. 

261.  Special  Steels.     Steels  may  contain  amounts  of  car- 
bon between  0  and  1.5  per  cent.     In  general,  the  higher  the 
carbon,  the  harder  the  steel,  but  the  heat  treatment  (temper- 
ing, annealing,  etc.)  has  a  great  effect  in  determining  the  hard- 
ness. 

High  carbon  steel  for  tools,  watch  springs,  razor  blades, 
and  the  like  requires  special  treatment  arid  is  made  on  a 


250 


METALLURGY 


much  smaller  scale  and  by  a  more  expensive  process  than 
common  steel. 

Of  the  other  elements  than  carbon  added  to  give  special 
properties  may  be  mentioned  chromium,  which  gives  a  very 
hard  steel  used  in  making  burglar-proof  safes,  and  nickel, 
which  gives  a  tough  steel  used  for  the  armor  plate  of  warships. 

WROUGHT  IRON 

262.  The  process  by  which  most  of  the  wrought  iron  is 
made  is  called  the  puddling  process.  The  furnace  used  re- 
sembles slightly  the  open  hearth  steel  furnace,  but  it  has  a 
fire  box  at  one  end  where  coal,  oil,  or  gas  is  burned  and  the 
flame  passes  over  the  charge  to  the  flue  at  the  other  end. 


FlG.  46.  —  Reverberatory  Furnace. 

This  type  of  furnace  is  called  a  reverberatory  furnace  because 
the  roof  is  so  constructed  as  to  deflect  or  reverberate  the  heat 
of  the  flame  down  on  the  charge.  Modifications  of  it  are  used 
in  various  other  metallurgical  processes. 

The  hearth  of  the  puddling  furnace  is  lined  with  blocks  of 
oxide  iron  ore.     The  pig  iron  is  usually  charged  in  the  solid 


WROUGHT  IRON  251 

form  and  melted  in  the  furnace.  The  oxygen  of  the  lining  and 
of  the  atmosphere  unites  with  the  impurities  in  the  charge, 
the  carbon  passes  off  as  carbon  monoxide,  and  the  other 
oxides  combine  to  form  slag.  The  iron,  which  is  now  nearly 
pure,  has  a  much  higher  melting  point  than  the  original  pig 
iron  and  instead  of  being  liquid  is  in  a  pasty  condition.  The 
men  in  charge  of  the  furnace,  with  tools  made  for  the  purpose, 
now  stir  the  charge,  mixing  slag  and  iron  together  and  then 
divide  it  into  several  balls  which  can  be  removed  from  the 
furnace  with  tongs  suspended  from  a  traveling  pulley. 
These  balls  are  passed  through  machines  which  squeeze 
out  most  of  the  slag  and  finally  roll  the  iron  out  into  the  rods 
and  bars  of  commerce. 

263.  Uses  of  Wrought  Iron  and  Very  Low  Carbon  Steel. 
Pure  iron,  unlike  the  impure  material,  corrodes  but  slowly. 
It  is  rather  soft,  being  easily  scratched  with  a  knife,  highly 
ductile  and  malleable,  but  still  it  has  a  fair  degree  of  tenacity. 
It  is  very  useful  where  great  toughness  combined  with  con- 
siderable strength  is  required. 

Wrought  iron  is  almost  pure  iron.  It  softens  sufficiently 
for  welding  below  1000  C.  although  it  does  not  melt  below 
1500°  C.  In  this  it  is  very  different  from  cast  iron,  which  does 
not  soften  before  it  melts.  Wrought  iron  is  therefore  suitable 
for  forging  —  it  is  the  metal  used  by  the  blacksmith  —  and 
the  slight  admixture  of  slag  that  it  contains  gives  it  a  fibrous 
structure  which  seems  to  increase  rather  than  detract  from 
its  usefulness  for  forging.  Wrought  iron  probably  resists 
corrosion  better  than  any  other  form  of  iron  and  this  property 
seems  to  be  due  to  its  freedom  from  carbon. 

Wrought  iron,  however,  is  more  costly  than  mild  steel  and  in 
recent  years  a  steel  unusually  low  in  carbon,  sometimes  called 
"  ingot  iron,"  has  been  prepared  in  large  amounts  and  is 


252  METALLURGY 

coming  into  use  for  wire  fences  and  many  other  objects  which 
must  have  a  high  resistance  to  corrosion  when  exposed  to 
the  weather.  It  is  a  fact  of  common  knowledge  that  some 
wire  fences  resist  corrosion  for  years,  whereas  others  are 
badly  corroded  in  a  few  months.  The  difference  can  almost 
always  be  traced  to  the  quality  of  the  iron. 

COPPER 

264.  Native  copper  is  found  abundantly  in  the  Lake  Su- 
perior mines  in  northern  Michigan.  It  is  also  found  in  some 

other  places,  but  not 
in  workable  quanti- 
ties. The  rock  in 
which  the  native  cop- 
per is  imbedded  is 
crushed,  and  the  cop- 
per particles  are  sep- 
arated from  the  rock 

FIG.  47.  —  Native  Copper. 

by  a  washing  process 

which  is  based  on  the  higher  specific  gravity  of  the  copper. 
The  metal  is  then  melted  and  cast  into  ingots. 

Copper  is  found  in  nature  principally  as  the  sulphide,  and 
it  is  usually  associated  with  iron  sulphide  and  other  sulphides. 
The  upper  layers  of  great  copper  deposits  are  often  found  to 
consist  of  the  oxide  and  the  carbonate,  but  as  the  mines  are 
deepened  the  ore  always  changes  to  sulphide.  The  oxygen 
and  carbon  dioxide  in  the  air,  assisted  by  percolating  waters, 
acting  for  thousands  of  years,  have  worked  down  to  some 
depth  and  slowly  oxidized  the  original  sulphide. 

Copper  is  obtained  from  the  sulphide  ores  in  two  steps. 
The  first  step  is  to  eliminate  all  of  the  waste  rock  and  part 
of  the  iron  and  sulphur,  thus  producing  copper  matte,  a 


COPPER 


253 


FIG.  48.  —  Chalcopyrite. 


mixture  o^  the  sulphides  of  copper  and  iron.  This  may  con- 
tain varying  amounts  of  copper  up  to  80  per  cent,  but  usually 
between  20  and  40  per 
cent.  The  next  step 
is  to  oxidize  all  the  re- 
maining iron  and  sul- 
phur, leaving  metallic 
copper. 

The  first  step  may 
be  performed  either  in 
a  reverberatory  fur- 
nace or  in  a  blast 
furnace.  The  latter 
resembles  the  iron 
blast  furnace,  but  is 
much  smaller.  The  second  step  is  to  treat  the  molten 
matte  in  a  converter  similar  to  the  Bessemer  steel  converter, 
except  that  instead  of  being  admitted  at  the  bottom  the 
air  is  blown  in  at  one  side  a  short  distance  above  the 
bottom.  The  oxygen  of  the  air  unites  with  the  sulphur 
and  the  iron;  the  sulphur  dioxide  passes  off  and  the  iron 
oxide  unites  with  the  silicious  lining  of  the  converter,  or  with 
silicious  material  thrown  into  the  matte,  and  forms  slag. 
When  the  impurities  are  oxidized  as  far  as  possible,  the  slag 
is  poured  off  and  the  copper  cast  into  cakes  for  refining. 

Oxide  ores  may  be  reduced  directly  to 'metallic  copper  in 
the  blast  furnace. 

265.  Refining  of  Copper.  The  copper  obtained  from  the 
furnaces  invariably  contains  other  metals,  including  gold  and 
silver.  The  most  important  use  of  copper  is  as  a  conductor 
of  electricity.  Even  small  amounts  of  such  good  conductors 
as  gold  and  silver  alloyed  with  copper  impair  its  conductivity ; 


254 


METALLURGY 


and  other  elements,  notably  arsenic,  impair  the  conductivity 
to  a  still  greater  degree. 

Crude  copper  may  be  refined  either  in  a  reverberatory  fur- 
nace or  by  an  electrolytic  method.  The  former  is  an  oxidiz- 
ing process  which  slags  most  of  the  impurities  and  is  used 
when  the  arsenic  and  precious  metals  are  low;  the  latter 
are  not  as  easily  oxidized  as  the  copper  itself.  The  electro- 
lytic method  removes  practically  all  of  the  impurities ;  and 
since  the  gold  and  silver  can  be  saved,  it  is  always  used 
when  there  is  a  sufficient  quantity  of  these  metals  present  to 
warrant  the  extra  expense  of  the  process.  The  main  opera- 
tions are  as  follows : 

The  copper  is  cast  in  plates  about  two  inches  thick  and 
four  feet  square  which  are  hung  alternately  as  positive  elec- 
trodes in  a  bath  con- 
taining copper  sul- 
phate and  sulphuric 
acid.  Alternating  in 
the  bath  with  these 
plates  are  thin  plates 
of  pure  copper,  all 
connected  with  the 
negative  conductor  of 
the  electric  circuit. 
In  Figure  49  we  have 
shown  five  negative 
and  four  positive 
plates.  In  one  of  the  large  refining  works,  they  put  twenty- 
three  negative  and  twenty-two  positive  plates  in  one  tank. 
One  building  has  sixteen  hundred  of  these  tanks  in  opera- 
tion. The  current  in  passing  dissolves  copper  from  the 
positive  electrode,  takes  it  through  the  solution,  and  deposits 


FIG.  49.  —  Electrolytic  Refining  of  Copper. 


LEAD 


255 


it  upon  the  negative  plates.  The  impurities  either  do  not 
dissolve  at  all  or  else  they  pass  into  solution  and  remain  in 
the  solution.  The  insoluble  impurities,  among  which  are  all 
the  gold  and  silver,  simply  drop  to  the  bottom  of  the  tank  as 
the  electrode  is  eaten  away.  The  mud  from  the  bottom  is 
worked  up  for  gold  and  silver  and  yields  a  valuable  profit. 

266.  Uses  of  Copper.     Copper  is  ductile,  tough,  and  resist- 
ant to  corrosion,  and  it  has  the  highest  electrical  conductivity 
of  any  metal  except  silver.     It  is  used  to  a  very  large  ex- 
tent in  electrical  work,  —  for  telephone,  electric  light,  and 
trolley  wires,  and  for  the  coils  of  all  electrical  machines. 

Its  resistance  to  corrosion  makes  it  a  useful  metal  for  ex- 
posed corners  and  edges  of  roofs,  for  water  conductors,  and 
cooking  utensils. 

Copper  is  an  important  component  of  many  useful  alloys, 
notably  brass  and  bronze.  Brass  usually  contains  about 
two  thirds  copper  and  one  third  zinc. 

LEAD 

267.  By  far  the  commonest  ore  of  lead  is  galena,  or  lead 
sulphide.     It    is    a 

beautifully  lustrous 
gray  crystalline  sub- 
stance, and  it  is  often 
found  crystallized  in 
almost  perfect  cubes. 

The  metallurgical 
treatment  of  lead  sul- 
phide is  similar  in 
principle  to  that  of 
copper  sulphide.  One 
method  is  to  roast  it  ~FIG.  50.  —  Galena. 

B.   AND  W.  CHEM. 17 


256  METALLURGY 

in  a  reverberatory  furnace  so  as  to  burn  the  sulphur  and 
leave  uncombined  lead.  The  process  is  not  carried  out, 
however,  in  a  single  operation.  It  is  usually  roasted  first 
with  free  access  of  air  whereby  only  a  part  of  the  ore  is  oxi- 
dized. Part  of  the  lead  sulphide  so  attacked  is  changed  to 
lead  sulphate  and  the  other  part  to  lead  oxide  and  sulphur 
dioxide. 

PbS  +  2  O2  ->  PbS04, 
2  PbS  +  3  O2  ->•  2  PbO  +  2  SO2. 

The  air  is  then  cut  off  and  the  temperature  is  raised,  whereby 
the  unchanged  lead  sulphide  reacts  with  the  lead  sulphate 
and  lead  oxide,  yielding  metallic  lead  and  sulphur  dioxide. 

PbSO4  +  PbS  ->  2  Pb  +  2  SO2, 
2  PbO  +  PbS  ->  3  Pb  +  SO2. 

Another  method  is  to  treat  the  ore  with  coke  and  fluxes 
in  a  blast  furnace.  The  products  of  this  furnace  are  slag, 
lead  matte,  and  metallic  lead. 

The  crude  lead  obtained  by  either  of  the  above  methods 
may  carry  considerable  amounts  of  silver  and  gold,  in  which 
case  it  is  refined  by  special  processes  designed  to  save  these 
metals.  The  amount  of  silver  obtained  in  this  way  is  of 
great  importance. 

268.  Uses  of  Lead.  Lead,  owing  to  its  high  degree  of 
resistance  to  the  action  of  water  and  of  some  acids,  is  used 
for  water  pipes,  coverings  of  wire  cables  in  conduits,  tanks 
and  containers  for  various  liquids,  and  for  the  chambers  in 
which  sulphuric  acid  is  made. 

Lead  salts  are  extremely  poisonous,  and  the  use  of  lead  for 
conveying  drinking  water  is  open  to  objection.  Although 
no  harmful  amount  is  dissolved  by  hard  water,  nevertheless 


ZINC  257 

soft  water,  particularly  rain  water,  dissolves  sufficient  lead 
oxide  from  the  pipes  to  cause  serious  danger. 

Lead  melts  very  easily ;   alloyed  with  tin  it  makes  solder. 

With  antimony  lead  forms  the  alloy  called  type  metal. 

The  great  density  of  lead  early  led  to  its  use  in  bullets  and 
shot. 

Large  amounts  are  also  made  into  white  lead,  a  basic  car- 
bonate of  approximately  the  composition  Pb(OH)2  •  2  PbCO3, 
which  is  used  in  paints. 

ZINC 

269.  The  most  important  ore  of  zinc  is  zinc  sulphide. 
This  is  roasted,  whereby  it  is  changed  to  oxide. 

2  ZnS  +  3  O2  ->  2  ZnO  +  2  SO2. 

The  sulphur  dioxide  given  off  is  commonly  utilized  to  make 
sulphuric  acid,  as  indeed  is  also  sometimes  done  at  lead  and 
copper  smelters.  The  zinc  oxide  is  mixed  with  crushed 
coal  and  heated  in  retorts. 

ZnO  +  C  ->  Zn  +  CO. 

Zinc  is  a  volatile  metal  when  compared  with  copper,  lead,  and 
iron.  Zinc  vapor  consequently  passes  from  the  retort  to- 
gether with  the  carbon  monoxide.  It  is  condensed  to  liquid 
zinc,  which  is  run  into  molds  to  solidify. 

Some  ores  which  are  not  suitable  for  the  production  of 
metallic  zinc  are  heated  on  a  grate  with  coal,  and  the  gases 
after  cooling  are  filtered  through  cloth.  The  zinc  is  thus  re- 
covered as  zinc  oxide,  a  white  powder,  which  has  a  ready 
sale  to  paint  and  rubber  manufacturers. 

270.  Uses  of  Zinc.     Zinc  is  one  of  the  most  active  of  the 
heavy  metals,  as  is  evidenced  by  the  vigor  with  which  it 


258  METALLURGY 

displaces  hydrogen  from  acids.  Hence  arises  its  use  in 
battery  cells  such  as  are  used  for  ringing  electric  bells.  It  is 
the  zinc  rod  or  plate  in  such  cells  which  is  eaten  away. 

In  view  of  the  great  chemical  activity  of  zinc  it  may  seem 
strange  that  it  is  so  little  attacked  by  the  weather,  and  that 
it  is  used  to  galvanize  iron  and  thus  to  protect  the  less  active 
metal.  The  secret  of  its  resistance  to  corrosion  is  that  it 
quickly  becomes  coated  with  a  thin  impervious  layer  of  oxide, 
which  excludes  air  and  dampness  from  the  metal  beneath. 

MERCURY 

271.  The  sulphide  ore  of  mercury  is  roasted  in  furnaces 
with  access  of  air.     The  sulphur  burns  to  sulphur  dioxide, 
and   the  mercury,  which   is  but  an  inactive  metal,  is  left 
uncombined.     It  distills  with  the  heat,  and  mercury  vapor 
passes  with  the  gases  into  a  condensing  chamber  from  which 
the  liquid  mercury  is  obtained. 

SILVER 

272.  A  large  part  of  the  world's  supply  of  silver,  as  already 
noted,  is  obtained  as  a  by-product  from  the  production  of 
lead  and  copper. 

Apart  from  some  native  silver,  the  important  ores  which 
are  worked  for  silver  alone  are  the  sulphide,  arseno-sulphide, 
and  the  chloride.  Many  different  processes  are  employed 
for  treating  silver  ores ;  the  one  to  be  described  is  perhaps 
typical. 

The  crushed  rock  containing  sulphide  ore  is  mixed  with 
common  salt  and  roasted,  whereby  the  silver  sulphide  is 
changed  to  chloride.  Silver  chloride  is  insoluble  in  water 
but  is  soluble  in  a  dilute  solution  of  sodium  thiosulphate. 


GOLD 


259 


The  roasted  sulphide  ore,  or  the  chloride  ore  direct,  is  treated 
with  sodium  thiosulphate  solution  and  the  liquid  is  then 
run  into  tanks  and  the  silver  precipitated  as  sulphide  by 
adding  sodium  sulphide.  The  insoluble  silver  sulphide  is 
recovered  by  filtering,  and  silver  is  obtained  from  this  by 
burning  off  the  sulphur. 

The  cyanide  process  as  described  under  gold  is  also  exten- 
sively used  for  silver  ores. 

GOLD 

273.  Gold  is  usually  found  in  the  uncombined  condition, 
but  in  exceedingly  small  particles  disseminated  throughout 
masses  of  rock.  Most 
of  the  gold  can  be 
recovered  by  finely 
crushing  the  ore  in 
stamping  machines 
and  washing  it  with 
water  through  a  shal- 
.  low  trough,  the  wide 
flat  bottom  of  which 
consists  of  a  copper 
plate  coated  with  mercury.  When  the  gold  comes  in  con- 
tact with  the  mercury,  it  amalgamates  with  it,  while  the 
waste  material  passes  on.  The  plates  are  scraped  periodi- 
cally and  the  gold  is  recovered  from  the  amalgam  by  distill- 
ing off  the  mercury. 

Some  ores  can  be  more  successfully  treated  by  a  chemical 
method.  The  one  most  used  is  the  cyanide  process,  in  which 
the  finely  crushed  ore  is  treated  with  a  solution  of  potassium 

I  cyanide  which,  together  with  oxygen  of  the  air,  reacts  with 
the  gold,  producing  a  soluble  compound.  The  solution  is  run 


FIG.  51.  — Native  Gold. 


260  METALLURGY 

into  tanks  filled  with  zinc  shavings.  It  has  already  been 
noted  that  zinc  is  a  more  active  metal  than  gold.  Here  again 
this  fact  is  shown,  for  the  zinc  passes  into  the  solution,  forcing 
the  gold  to  precipitate  as  free  metal.  This  finely  divided 
gold  is  then  melted  and  cast  into  bars. 

ALUMINIUM 

274.  Aluminium  oxide  cannot  be  reduced  to  metallic  alu- 
minium by  means  of  carbon  in  a  blast  furnace  because  of  the 
very  great  chemical  activity  of  the  element.  Aluminium 
oxide  will  react  with  carbon  at  a  very  high  temperature,  but 
aluminium  carbide  and  not  the  uncombined  metal  is  ob- 
tained. 

Formerly  aluminium  was  obtained  by  treating  aluminium 
chloride  with  sodium, 

3  Na  +  Aids  ->  3  Nad  +  Al, 

but  this  method  is  now  entirely  superseded  by  the  electrolytic 
method. 

Aluminium  oxide  can  be  melted  only  at  a  very  high 
temperature,  but  cryolite,  a  mineral  of  the  formula 

_____ 3  NaF  •  A1F3,    melts 

very  easily,  and,  fur- 
thermore, will  dis- 
solve aluminium 
oxide  much  as  water 
will  dissolve  common 
salt.  In  the  elec- 


FIG.  52.  —  Electrolytic  Production  of  Aluminium. 

trolytic  process  cry- 
olite is  melted  in  iron  vats  lined  with  carbon,  aluminium 
oxide  is  added,  and  carbon  electrodes  are  introduced.  The 
current  enters  through  the  carbon  electrodes  and  passes  out 


ALUMINIUM  261 

through  the  iron  vats.  The  resistance  to  the  passage  of 
the  current  causes  heating  sufficient  to  keep  the  charge 
melted.  Oxygen  is  liberated  at  the  carbon  electrode  and 
either  escapes  or  burns  the  carbon  to  carbon  monoxide. 
The  aluminium  is  liberated  at  the  surface  of  the  carbon 
lining  and  sinks  in  the  melted  condition  to  the  bottom  of 
the  bath,  where  it  is  occasionally  tapped  off.  The  process 
is  continuous  and  fresh  aluminium  oxide  is  added  as  fast 
as  that  in  the  bath  is  decomposed.  The  cryolite  suffers  no 
change. 

The  chief  ore  of  aluminium  is  bauxite,  a  rather  impure 
aluminium  oxide  containing  water.  For  the  best  grade  of 
metal,  this  ore  must  be  purified  chemically  so  that  only  pure 
aluminium  oxide  shall  be  introduced  into  the  electrolytic 
bath. 

275.  Uses  of  Aluminium.  Aluminium  is  the  lightest  of 
the  metals  which  are  strong  enough  and  resistant  enough 
to  corrosion  to  be  useful  for  objects  to  stand  wear  and 
weather.  The  resistance  to  corrosion  does  not  arise,  however, 
from  its  chemical  inactivity,  but  rather  from  the  fact  that  it 
coats  itself  with  a  very  impervious  film  of  oxide.  The  truth 
of  this  statement  is  shown  by  an  interesting  experiment. 
If  a  piece  of  aluminium  is  carefully  cleaned  with  hydrochloric 
acid  and  a  little  mercury  is  rubbed  on  the  surface,  it  amalga- 
mates. Soon  a  white  fluffy  powder,  aluminium  oxide,  is  seen 
to  form  over  the  amalgamated  surface  and* to  increase  quite 
rapidly  in  bulk.  Before  long  the  aluminium  has  entirely 
disappeared,  a  little  globule  of  mercury  is  left,  and  a  heap  of 
the  fluffy  powder.  On  the  surface  of  the  amalgam  the 
aluminium  oxidizes  as  well  as  on  the  surface  of  the  pure  metal, 
but  the  oxide  does  not  adhere  to  the  liquid  surface ;  it  floats 
off  and  therefore  the  oxidation  continues. 


262  METALLURGY 

Aluminium  is  not  as  cheap  as  steel,  and  its  use  is  confined 
mostly  to  articles  in  which  lightness  is  a  prime  essential, 
such  as  parts  of  automobiles  and  flying  machines.  Alumin- 
ium serves  excellently  for  cooking  utensils.  It  is  a  very  good 
conductor  of  heat  and  electricity  and  it  is  sometimes  used  in 
place  of  copper  for  electric  conductors. 

SUMMARY 

Metals  in  Nature :  All  metals  except  the  chemically  inactive  ones, 
such  as  copper,  silver,  gold,  and  platinum,  are  found  in  nature 
always  in  combination  with  non-metals,  most  often  with 
oxygen  and  sulphur.  Gold  and  platinum  are  almost  always, 
and  silver  and  copper  are  sometimes,  found  uncombined. 

Metals  from  Ores :  Oxygen  can  be  removed  from  combination  with 
most  of  the  heavy  metals  by  means  of  carbon,  for  the  heavy 
metals  are,  in  the  main,  not  very  active  chemically. 

The  sulphides  can  be  converted  into  oxides  by  roasting,  whereby 
sulphur  dioxide  escapes. 

Oxides  of  the  very  active  metals  like  aluminium,  sodium,  potas- 
sium, magnesium,  and  calcium  cannot  be  reduced  with  carbon 
and  obtained  as  uncombined  metals.  Recourse  is  usually  had 
in  these  cases  to  electrolysis  of  their  fusible  compounds. 

Questions 

1.  Suggest  a  method  to  obtain  metallic  tin  from  tinstone. 

2.  What  metals  can  be  obtained  from  their   oxide  ores  by  re- 
duction with  carbon  ?     What  ones  cannot  ? 

3.  What  method  is  available   for    separating   the  metal  when 
the  oxide  cannot  be  reduced  with  carbon? 

4.  How  might  mercury  be  obtained  from  mercuric  oxide  with- 
out using  carbon  ? 

6.  Explain  how  aluminium  oxide  is  brought  into  the  liquid 
condition  previous  to  the  separation  of  the  metal  by  electrolysis. 

6.  How  are  gold  and  silver  extracted  from  the  crude  copper 
from  the  smelter? 


QUESTIONS  263 

7.  When  refuse  rock  from  copper  mines  is  heaped  up  and  kept 
moist,  any  copper  sulphide  present  oxidizes  slowly  to  copper  sul- 
phate, which  is  soluble.     How  might  metallic  copper  be  obtained 
from  the  solution  draining  from  the  bottom  of  the  heap  ? 

8.  Calculate  the  per  cent  of  iron  in  each  of  the  four  minerals, 
Fe203,  Fe304,  FeC03,  and  Fe203  •  3  H2O. 

9.  What  weight  of  carbon  would  be  necessary  to  reduce  (a)  1000 
kilograms  of  cuprous  oxide,  Cu20,   (6)  1000  kilograms  of  cupric 
oxide,  CuO,  assuming  the  carbon  is  all  changed  to  carbon  dioxide 
in  the  process  and  neglecting  the  amount  of  fuel  which  must  be 
burned  to  supply  heat. 

10.  What  weight  of  carbon  is  needed  to  reduce  1000  kilograms 
of  zinc  oxide?     The  carbon  is  oxidized  only  to  carbon  monoxide. 


CHAPTER   XXIII 

COMPOUNDS    OF    CARBON 

WE  have  already  become  familiar  with  some  of  the  prop- 
erties and  some  of  the  compounds  of  carbon.  We  have 
seen  that  carbon  enters  into  a  continuous  cycle  of  changes 
between  living  and  inorganic  matter.  Fuels,  —  wood,  coal, 
charcoal,  —  which  are  without  exception  the  product  of 
present  or  past  organic  life  burn,  and  the  carbon  in  them 
yields  the  gas  carbon  dioxide.  Plants  take  carbon  dioxide 
from  the  air  through  the  surfaces  of  their  green  leaves  and 
with  the  aid  of  the  energy  obtained  from  sunlight  convert 
it  into  the  most  complex  and  varied  compounds  containing 
sometimes  oxygen  and  hydrogen  and  sometimes  nitrogen  in 
addition.  These  compounds  in  time  always  yield  carbon 
dioxide  back  again  to  the  air,  for  they  are  either  burned 
directly  as  fuel,  or  they  are  eaten  by  animals  and  burned 
slowly  in  the  animal  tissues,  or  they  decay,  which  is  also  a 
process  of  slow  combustion. 

In  the  present  chapter  we  shall  deal  mainly  with  carbon 
as  it  exists  in  substances  which  are  not  closely  related  to 
living  things.  In  Chapter  XXIV,  a  brief  view  will  be 
given  of  a  few  of  the  more  important  of  the  compounds 
of  carbon,  which  are  formed  like  starch  and  sugar  directly 
in  life  processes,  or  are  at  least  derived  from  compounds  so 
formed. 

264 


AMORPHOUS   CARBON  265 

UNCOMBINED  CARBON 

There  are  three  different  forms  in  which  uncombined  carbon 
exists,  namely,  amorphous  carbon,  which,  as  the  name  im- 
plies, is  without  definite  form  or  shape,  and  two  different 
crystalline  forms,  graphite  and  diamond. 

276.  Amorphous  carbon  is  without  crystalline  structure 
and  in  that  sense  it  is  without  form.  Masses  of  it  may,  of 
course,  exist  in  any  shape,  but  there  is  no  evidence  within 
the  mass  of  any  tendency  to  a  definite  structure.  Amorphous 
carbon  is  obtained  when  organic  matter  is  decomposed  by 
great  heat.  The  carbon  is  left  as  a  charred  mass,  while  the 
other  elements  of  the  compound  are  driven  off  as  volatile 
matter.  Charcoal,  coke,  lampblack,  and  gas  carbon  are 
well-known  varieties  of  amorphous  carbon. 

Charcoal  is  obtained  by  heating  wood  in  the  absence  of 
air ;  a  number  of  volatile  products,  including  water,  wood 
alcohol,  acetic  acid,  acetone,  and  tar,  are  driven  off,  and  with 
these  products  escape  all  of  the  hydrogen  and  oxygen  as  well 
as  a  portion  of  the  carbon  of  the  wood,  while  charcoal  is  left 
as  a  non- volatile  residue.  Charcoal  is  nearly  pure  carbon, 
except  that  it  still  contains  the  mineral  matter  of  the  wood. 
All  ordinary  kinds  of  charcoal,  therefore,  leave  a  little  mineral 
ash  when  they  are  burned. 

Coke  is  obtained  from  soft  coal  in  much  the  same  manner 
as  charcoal  is  obtained  from  wood.  The 'volatile  products 
driven  off  are  mainly  illuminating  gas,  coal  tar,  and  ammonia. 

Lampblack  is  familiar  to  all  who  have  seen  a  smoking 
kerosene  lamp.  It  is  manufactured  by  allowing  petroleum 
oil  or  natural  gas  to  burn  with  an  insufficient  supply  of  air. 
The  heat  of  the  flame  decomposes  the  compound  ;  the  hydro- 
gen burns  and  maintains  the  flame,  while  a  large  part  of  the 


266  COMPOUNDS  OF  CARBON 

carbon  remains  in  extremely  small  particles  as  soot  or  lamp- 
black. This  adheres  to  a  cooled  surface  which  continually 
revolves  just  above  the  flame.  A  scraper  bears  upon  another 
part  of  the  revolving  surface  and  scrapes  off  the  lampblack 
into  an  appropriate  receptacle.  Lampblack  is  much  used  as 
a  pigment  in  paints  and  especially  in  printer's  ink. 

Gas  carbon  is  formed  in  the  retorts  in  which  soft  coal  is 
destructively  distilled  in  the  manufacture  of  illuminating 
gas.  It  is  formed  where  the  gas  comes  in  contact  with  the 
white-hot  top  and  sides  of  the  retort  and  is  there  decomposed. 
Being  formed  at  a  higher  temperature,  it  is  harder  and  more 
compact  than  lampblack.  It  is  largely  used  in  the  manu- 
facture of  electric-light  carbons. 

277.  Coal.     Coal  is  not  pure  carbon,  but  it  consists  very 
largely  of  carbon.     It  is  a  product  of  the  decomposition  of 
trees  and  plants  which  grew  in  a  long-past  geologic  age.     By 
floods  or  other  natural  agencies,  the  luxuriant  vegetation  of 
the  so-called  carboniferous  period  of  the  earth's  history  be- 
came covered  with  deep  layers  of  earth  in  some  localities 
before  it  had  a  chance  to  fully  decay.     Deep  down  beneath 
the  earth's  surface,  heat  and  pressure  have  in  the  course  of 
centuries  changed  the  original  organic  material  into  coal. 
Coal  still  contains  some  of  the  hydrogen,  oxygen,  and  nitro- 
gen of  the  original  organic  matter ;   soft  or  bituminous  coal 
contains  a  considerable  amount ;    hard  or  anthracite  coal 
contains  much  less. 

278.  Graphite.     We  have   seen  above  that  the   residue 
obtained  by  artificially  heating  coal  in  coking  ovens  is  coke, 
one  of  the  varieties  of  amorphous  carbon.     The  final  product 
of  a  similar  process  in  nature  is  graphite.     Where  coal  has 
been  very  deeply  buried  and  consequently  subjected  to  more 
of  the  earth's  interior  heat,  as  well  as  enormous  pressure,  the 


GRAPHITE  267 

decomposition  has  been  more  complete.  The  result  of  this 
decomposition  when  carried  to  its  final  stage  is  graphite. 

Graphite  is  found  in  nature  crystallized  in  soft  hexagonal 
plates  or  prisms.  Its  specific  gravity  is  2.25 ;  it  is  consider- 
ably more  dense  than  amorphous  carbon.  It  is  exceedingly 
soft  and  offers  but  little  friction  when  rubbed  ;  on  account  of 
this  property,  it  is  much  used  as  a  lubricant.  It  is  black  and 
opaque  and  it  conducts  electricity.  It  is  much  used  in  elec- 
trical work  on  account  of  its  conductance  and  its  low  coeffi- 
cient of  friction. 

The  most  familiar  use  of  graphite  is  in  "  lead  "  pencils, 
for  which  purpose  it  is  mixed  with  clay  to  give  the  desired 
degree  of  hardness  and  then  molded  into  the  "  leads." 

Graphite  is  very  resistant  to  heat,  so  that  it  is  an  excellent 
material  for  making  crucibles,  electrodes,  and  linings  in  fur- 
naces in  which  high  temperatures  are  employed.  It  is  ex- 
tremely hard  to  make  graphite  burn.  The  harder  an  anthra- 
cite coal,  the  more  difficulty  there  is  in  getting  it  to  burn, 
and  when  the  coal  is  graphitic  in  nature,  it  scarcely  burns  at 
all  in  a  furnace. 

In  pure  oxygen,  a  complete  combustion  of  graphite  can  be 
effected,  and  the  only  product  of  the  combustion  is  carbon 
dioxide.  It  is  thus  made  certain  that  graphite  is  composed 
only  of  carbon. 

Graphite  is  now  made  artificially  in  an  electric-furnace 
process  in  which  an  enormous  current  is  Sent  through  a  fur- 
nace packed  with  anthracite  coal.  The  current  produces 
the  temperature  of  the  electric  arc;  the  coal  is  completely 
decomposed  and  all  the  ordinarily  volatile  matter  and  even 
the  mineral  matter,  which  in  an  ordinary  furnace  is  involatile, 
are  expelled  as  vapor.  Carbon  alone  is  left  as  a  residue,  and 
this  is  changed  into  the  form  of  graphite. 


268 


COMPOUNDS  OF   CARBON 


279.  Diamond  is  the  most  beautiful  and  the  rarest  as  well 
as,  perhaps,  the  most  interesting  form  of  carbon.  It  is  just 
as  much  pure  carbon  as  are  graphite  and  amorphous  carbon, 
as  can  be  proved  by  burning  it  in  oxygen ;  it  gives  only  car- 
bon dioxide,  and  a  given  weight  of  diamond  gives  exactly 
the  same  amount  of  carbon  dioxide  as  the  same  weight  of 
either  of  the  other  forms  of  carbon. 

One  would  never  suspect  from  their  physical  properties 
that  diamond  and  graphite  are  both  composed  entirely  of 
the  same  element,  for  no  two  substances 
could    be  more  dissimilar.      Diamond  is 
the    hardest    substance    known,    whereas 
graphite  is  one  of  the  softest  of  solid  sub- 
stances.    Diamond  is   colorless,  transpar- 
ent,  and   a  non-conductor  of  electricity, 
whereas  graphite  is  black,  opaque,  and  a 
conductor  of  electricity.     The  diamond  is 
insoluble  in  all  known  liquids  under  ordi- 
nary conditions  of  temperature  and  pres- 
sure.    Diamond  crystallizes  in  the  regular 
system,    usually    in    octahedra,    whereas 
graphite  crystallizes  in  hexagonal  plates. 
Diamond  has  a  specific  gravity  of  3.51,  and  it  is  thus  by  far 
the  densest  form  of  carbon.     When  it  is  intensely  heated  in 
the  absence  of  air,  it  swells  and  becomes  black  graphite,  but 
when  heated  intensely  in  oxygen,  the  diamond  burns  com- 
pletely, producing  only  a  gaseous  product. 

The  differences  between  diamond,  graphite,  and  amorphous 
carbon  are  believed  to  be  due  to  the  same  causes  as  the  dif- 
ference between  ozone  and  ordinary  oxygen,  that  is,  to  a 
difference  in  the  number  of  atoms  in  the  molecule  and  possibly, 
also,  to  the  way  in  which  the  atoms  are  linked  together. 


FIG.  53.  —  Diamond. 

(Natural  Crystal.) 


DIAMONDS  269 

It  was  discovered  by  Moissan,  the  French  chemist,  that 
diamonds  of  microscopic  size  can  be  made  artificially  by  al- 
lowing carbon  to  crystallize  from  solution  in  a  molten  material 
kept  under  great  pressure.  Carbon  dissolves  to  a  consider- 
able extent  in  molten  iron.  If  a  mass  of  molten  iron  satu- 
rated with  carbon  is  suddenly  chilled,  the  outer  layers 
solidify,  while  the  inner  portion  remains  liquid.  The  latter 
afterwards  solidifies  more  slowly,  and  since  iron  containing 
much  carbon  expands  on  solidifying,  it  exerts  a  great  pressure 
against  the  already  solid  exterior  of  the  mass.  The  dissolved 
carbon  crystallizing  out  under  this  great  pressure  assumes 
the  form  of  diamond.  Under  ordinary  pressure,  the  carbon 
separates  as  graphite.  By  dissolving  away  the  iron  with 
acid,  one  can  obtain  the  tiny  diamond  crystals,  but  all  that 
have  ever  been  made  in  this  way  are  too  small  to  be  of  value. 
There  is  geological  evidence  that  the  natural  diamonds 
of  South  Africa  have  crystallized  from  solution  in  molten 
rock  subjected  to  enormous  pressure. 

280.  Uses  of  Diamonds.  Aside  from  its  beauty  when  cut 
as  a  gem,  diamond  is  of  use  in  the  industrial  arts  on  account 
of  its  great  hardness.  The  poorer  stones  that  cannot  be  made 

into  gems  are  set  in  solder 
on  the  edges  of  great  saws 
that  are  used  for  stone 
cutting ;  similarly,  they 
are  set 'at  the  bottom  of 
tubular  drills  that  are  used 

FIG.  54. -Diamond.    (Cut  Gem.)      "   for  making  deep  borings  in 

rock.  Diamonds  are  used 
in  the  jeweled  bearings  of  delicate  instruments  of  precision. 
Wires  of  exact  dimensions  are  made  by  drawing  the  metal 
through  perforations  in  thin  plates  of  diamond. 


270  COMPOUNDS  OF  CARBON 

CARBON  AND  OXYGEN 

We  have  already  seen  in  Chapter  IV  that  carbon  burns  in 
a  plentiful  supply  of  oxygen  to  form  carbon  dioxide,  but  with 
a  restricted  supply  it  forms  carbon  monoxide. 

281.  Carbon  dioxide  is  the  anhydride  of  carbonic  acid, 
H2CO3,  a  comparatively  weak  acid,  but  still  one  that  is  ca- 
pable of  forming  salts  with  all  strong  bases.     A  vast  amount 
of  carbon  dioxide  is  thus  held  in  the  mineral  world  in  com- 
bination  with   various   bases.     Calcium    carbonate    is   the 
most  plentiful  of  the  carbonates.     Limestone,  marble,  and 
shells  of  marine  animals  are  natural  forms  of  this  substance. 
Magnesium  carbonate  is  also  very  abundant ;    carbonates  of 
iron,  lead,  and  copper  are  abundant  and  constitute  valuable 
ores  of  these  metals. 

282.  Carbon  monoxide  is  not  acidic  in  nature,  as  is  the 
dioxide ;    it  therefore  does  not  combine  with  bases,  and  we 
never  find  it  fixed  as  a  component  of  any  of  the  solid  matter 
of  the  earth's  crust.     Its  most  striking  properties  are  its 
poisonous  character  and  its  combustibility. 

The  escape  of  carbon  monoxide  into  living  rooms,  either 
from  charcoal  fires,  from  faulty  furnaces,  or  from  illuminating 
gas  cocks,  is  a  source  of  grave  danger. 

Carbon  monoxide  is  so  combustible  and  yields  so  much 
heat  in  burning  that  in  spite  of  its  poisonous  character  great 
quantities  of  it  are  made  and  used  admixed  with  other  gases 
as  fuel.  Pure  carbon  monoxide  burns  with  a  pale  blue 
flame  and  yields  carbon  dioxide. 

283.  Water  gas  is  essentially  a  mixture  of  equal  volumes  of 
carbon  monoxide  and  hydrogen.    It  is  made  by  blowing  steam 
through  incandescent  carbon  with  which  it  reacts  according 
to  the  equation :     c  ^  CQ       ^ 


WATER  GAS 


271 


In  the  process  of  making  water  gas  for  lighting  in  large 
cities,  coke  or  anthracite  coal  is  placed  in  large  vertical  cylin- 
drical furnaces  (generators)  and  kindled  at  the  bottom.  Air 
is  blown  in  until  the  fire  has  heated  the  whole  mass  to  incan- 
descence. Then  the  air  is  cut  off  and  steam  is  blown  in  and 


FIG.  55.  —  Manufacture  of  Water  Gas. 

the  water  gas  is  produced.  When  the  furnace  has  become 
partly  cooled  by  this  last  process,  the  gas  formation  ceases, 
and  the  mass  must  be  again  heated  by  blowing  in  air. 

Since  hydrogen  and  carbon  monoxide  both  burn  with  nearly 
colorless  flames,  water  gas  cannot  be  used  by  itself  for  illumi- 
nating purposes.  Illuminants  are  usually  added  to  it  by 
"  cracking  "  petroleum  oil.  The  gas  is  led  through  a  chamber 
(the  carburetor)  containing  a  checkerwork  of  white-hot  fire 
brick  over  which  petroleum  oil  drops.  The  heat  decomposes 
the  oil  vapor  as  it  passes  through  the  superheater  and  per- 
manent gases  of  high  illuminating  power  are  thereby  formed. 

284.  Producer  Gas.  When  air  enters  from  the  bottom  of 
a  deep  anthracite  coal  or  coke  fire,  carbon  dioxide  is,  without 

B.   AND  W.  CHEM. 18 


272 


COMPOUNDS  OF  CARBON 


doubt,  formed  in  the  lower  layer.  As  this  gas  rises,  however, 
it  passes  through  other  layers  of  white-hot  carbon  and  is 
thereby  reduced  to  carbon  monoxide. 

C02+C->2CO. 

The  long,  pale  blue  flames  seen  rising  from  an  anthracite 
fire,  in  an  ordinary  house  furnace,  are  due  to  the  burning 
carbon  monoxide. 

The  heat  developed,  when  carbon  monoxide  burns  to  car- 
bon dioxide,  is  more  than  twice  as  great  as  that  set  free  when 
carbon  burns  to  carbon  monoxide.  It  is  obvious,  then,  that 
to  obtain  anything  like  the  full  heating  value  of  the  coal, 


FIG.  56.  —  Manufacture  of  Producer  Gas. 


enough  air  must  be  admitted  through  the  upper  door  of  the 
furnace  to  completely  burn  the  carbon  monoxide. 

Gas  that  is  formed  in  a  manner  similar  to  this  is  used  ex- 
tensively in  metallurgy  and  other  industries,  and  is  known 


COAL  GAS  273 

as  producer  gas.  Air  is  drawn  through  a  very  deep  coal 
fire  in  the  generator.  The  resulting  gas  mixture  contains 
all  the  nitrogen  of  the  air,  but  all  of  the  oxygen  has  com- 
bined with  carbon  to  form  carbon  monoxide.  The  gas  is 
conducted  through  pipes  to  the  furnaces  where  it  is  to  be 
burned  as  fuel.  By  the  process  as  so  far  outlined,  it  is  ap- 
parent that  the  heat  developed  by  the  addition  of  the  first 
unit  of  oxygen  to  the  carbon  would  be  wasted.  By  moist- 
ening the  in-going  air  as  it  passes  through  the  economizer  or 
by  introducing  a  carefully  regulated  quantity  of  steam  into 
the  furnace,  together  with  the  air,  this  heat  is  largely 
utilized  in  producing  water  gas.  The  addition  of  this  water 
gas  to  the  producer  gas,  of  course,  considerably  increases  the 
fuel  value  of  the  latter. 

285.  Coal  gas  is  made  by  the  destructive  distillation  of 
soft  coal.  The  coal  is  placed  in  fire-clay  retorts  or  in  ovens 
and  heated  to  a  white  heat.  The  chemical  compounds  exist- 
ing in  the  coal  are  mostly  broken  down  and  simpler  substances 
are  formed.  Coke  is  left  behind  while  a  great  variety  of  vol- 
atile substances,  containing  some  of  the  carbon  of  the  coal  and 
all  of  the  hydrogen,  are  driven  off.  After  purification  from 
tar,  ammonia,  sulphur  compounds,  etc.,  the  gas  is  ready  for 
delivery  to  the  pipes  of  the  city  lighting  system.  This 
purification  consists  mainly  in  condensing  all  substances 
which,  if  not  thus  removed,  would  later  condense  when  cooled 
in  the  gas  mains,  thus  tending  to  clog  them.  It  also  serves 
to  remove  certain  valuable  by-products,  principally  coal  tar 
and  ammonia.  The  process  in  addition  largely  removes 
hydrogen-sulphide  gas,  the  presence  of  which  in  the  illumi- 
nating gas  is  objectionable,  because  on  burning  it  produces 
sulphur-dioxide  gas,  which  is  harmful  to  people  and  to  plants, 
especially  when  formed  in  poorly  ventilated  houses.  The 


274 


COMPOUNDS  OF  CARBON 


condensation  of  the  less  volatile  materials,  such  as  tar,  takes 
place  in  a  series  of  condensers  and  scrubbers.  The  condensers 
may  be  either  air  or  water  cooled.  In  the  scrubbers  the  gases 
are  subjected  to  the  cooling  and  dissolving  action  of  water 
which  is  sprayed  over  a  grid  or  lattice-like  arrangement 
of  wooden  strips  in  a  rectangular  tower.  The  gases  are 
thoroughly  washed  by  contact  with  the  large  extent  of  wet 
surface.  Thus  much  tar  and  ammonia  are  removed  from  the 


FIG.  57.  —  Manufacture  of  Coal  Gas. 

crude  gas.  Rotary  scrubbers  are  also  sometimes  used  in 
which  the  crude  gas  is  churned  with  water  to  remove  tar 
and  ammonia  and  other  objectionable  materials. 

The  removal  of  the  hydrogen  sulphide  is  effected  by 
means  of  iron  rust,  in  the  following  fashion :  iron  turnings 
(to  give  great  surface)  are  mixed  with  wood  shavings  (to 
keep  the  mass  porous) ,  and  the  mixture  is  then  moistened  and 
the  iron  allowed  to  rust.  Large  flat  boxes  (the  purifiers) 
are  packed  with  the  mixture  and  sealed  tightly  by  water 
seals.  The  gas  is  then  led  through  the  mass,  when  the 
hydrogen  sulphide  reacts  with  the  hydrated  iron  oxide,  iron 


LUMINOSITY  OF  FLAME  275 

sulphide  and  water  resulting.  Thus  the  sulphur  compound 
is  removed  from  the  gas.  When  the  iron  rust  is  spent,  the 
mass  is  removed  from  the  box  and  exposed  to  the  action 
of  the  air,  whereby  the  iron  sulphide  is  oxidized  and  iron 
oxide  is  again  obtained.  The  mixture  can  thus  be  used  re- 
peatedly until  too  full  of  the  resulting  uncombined  sulphur 
for  efficient  results.  Coal  gas  contains  on  an  average  nearly 
50  per  cent  of  hydrogen  by  volume,  a  considerable  amount 
of  methane,  CH4,  and  smaller  amounts  of  ethylene,  C2H4, 
acetylene,  C2H2,  and  many  other  compounds  of  carbon  and 
hydrogen. 

286.  Luminosity  of  Flame.  As  a  rule,  hot  gases  are  not 
of  themselves  luminous.  Consequently,  a  flame  which  con- 
tains nothing  but  heated  gases,  as,  for  example,  the  hydrogen 
flame  or  the  carbon  monoxide  flame,  does  not  give  off  light. 

On  the  other  hand,  the  flame  of  burning  ethylene,  C2H4,  is 
brilliantly  luminous.  The  light  comes  from  solid  particles 
of  carbon  raised  to  incandescence  by  the  heat  of  the  flame. 
On  entering  the  hot  zone,  the  gas  is  decomposed  by  the  heat 
into  hydrogen  and  free  carbon.  The  hydrogen  burns  first 
with  the  limited  supply  of  air  that  reaches  the  interior  of  the 
flame,  while  the  minute  particles  of  solid  carbon  remain  sus- 
pended in  the  current  of  gas  and  give  the  light.  As  the  white- 
hot  carbon  reaches  the  exterior  of  the  flame,  it  comes  in  con- 
tact with  oxygen  and  it  too  burns. 

It  is  easy  to  prove  that  solid  carbon  exists  in  the  interior 
of  a  luminous  flame,  for  if  a  piece  of  cold  porcelain  is  thrust 
into  the  flame,  it  becomes  coated  with  soot  (lampblack).  The 
glowing  particles  of  carbon  are  cooled  by  contact  with  the 
porcelain  to  below  their  kindling  temperature  before  they 
get  a  chance  to  burn.  (See  Lampblack,  Sec.  276.) 

Ethylene  is  one  of  the  typical  illuminants  of  both  water 


276  COMPOUNDS  OF  CARBON 

and  coal  gas.  It  is  obtained  in  water  gas  by  the  cracking 
of  petroleum  oil,  whereas  in  coal  gas  it  results  naturally 
in  small  quantities  from  the  decomposition  of  the  coal. 
Another  and  more  powerful  illuminant  that  is  present  to  some 
extent  in  water  and  coal  gas  is  acetylene,  C2H2.  The  higher 
proportion  of  carbon  which  it  contains  is  responsible  in  part 
for  its  high  illuminating  power.  When  pure  acetylene  is 
burned,  it  gives  an  extremely  brilliant  light. 

287.  Welsbach  Mantles.     One  of  the  greatest  inventions 
in  the  field  of  gas  lighting  is  the  Welsbach  mantle.     It  is 
this  mantle  that  is  raised  to  incandescence  by  the  heat  of  the 
flame  rather  than  solid  particles  of  carbon  resulting  from  the 
decomposition  of  the  gas  itself.     Where  Welsbach  mantles 
are  used,  it  is  a  matter  of  indifference  whether  gas  is  rich  in 
illuminants  or  not.     Water  gas  can  then  be  used  without 
being  enriched  with  "  cracked  "  petroleum.     The  Welsbach 
mantles  are  composed  of  the  oxides  of  two  rare  metals,  tho- 
rium and  cerium. 

CARBIDES 

288.  Carbides.     It  has  already  been  seen  that  the  typi- 
cally non-metallic  elements,  oxygen,  chlorine,  and  sulphur, 
form  compounds  with  the  metals,  that  is,  oxides,  chlorides, 
and    sulphides,    respectively.     Carbon   is   less   distinctly   a 
non-metal  than  the  elements  mentioned.     It  does  not  unite 
energetically  with  metallic  elements,  and  the  carbides  when 
they  are  obtained  are  not  as  well-defined  compounds  as  the 
oxides,  chlorides,  and  sulphides.    The  carbides  of  iron,  silicon, 
and  calcium  are  worthy  of  mention  on  account  of  their  im- 
portance in  the  industrial  arts. 

289.  Iron  Carbide.     Molten  iron  is  capable  of  dissolving 
a  good  deal  of  carbon  in  much  the  same  manner  as  water 


CARBIDES 


277 


dissolves  sugar.  When  the  molten  mass  is  allowed  to  cool 
very  slowly,  the  carbon  separates  from  its  solution  in  the 
iron  and  appears  partly  as  flakes  of  graphite  and  partly  as 
microscopic  crystals  of  a  definite  carbide  of  iron,  Fe3C.  These 
flakes  of  graphite  and  crystals  of  iron  carbide  do  not  get  a 
chance  to  settle  out  or  to  float  off,  but  they  remain  suspended 
throughout  the  mass  of  metal  to  the  etched  surface  of  which 
they  give  a  mottled  appearance  under  the  microscope. 

If  the  melted  iron  is  cooled  very  rapidly,  none  of  the  carbon 
has  a  chance  to  separate  out,  but  it  still  remains  in  solution 
in  the  now  solid  iron  (solid  solution),  and  in  consequence  of 
the  presence  of  carbon  in  this  state  the  metal  is  extremely 
hard  as  well  as  brittle.  The  extreme  hardness  and  brittle- 
ness  may  be  removed  to  any  desired  extent  by  cautious 
reheating,  —  the  so-called  tempering  process.  During  the 
tempering  process  more  or  less  of  the  carbon  passes  from  the 
solid  solution  into  the  microscopic  crystals  of  iron  carbide. 

Iron  carbide  has  never  been  prepared  pure  in  large  masses ; 
it  is  only  known  in  the  form  of  the  infinitesimally  small  crys- 
tals embedded  in  masses  of  iron.  It  is,  nevertheless,  a  sub- 
stance of  great  im- 
portance in  view  of 
its  pronounced  effect 
upon  the  properties 
of  iron  and  steel. 

290.  Silicon  Car- 
bide, SiC.  This 
compound  of  carbon 
is  of  considerable 
practical  importance 
on  account  of  its  ex- 
treme  hardness, 


FIG.  58.  —  Making  Carborundum  in  an  Electric 
Furnace. 


278 


COMPOUNDS  OF  CARBON 


which  renders  it  of  great  value  as  a  grinding  material. 
It  is  known  in  the  trade  as  carborundum.  Grinding  wheels, 
sharpening  stones,  etc.,  are  made  from  powdered  car- 
borundum which  is  mixed  with  a  binding  material  and 
molded  into  shape.  Carborundum  is  made  by  heating 
quartz  sand  (SiO2)  with  coke  in  an  electric  furnace.  Part 
of  the  carbon  withdraws  oxygen  from  the  silicon  dioxide, 
while  at  the  same  time  another  part  of  the  carbon  unites  with 
the  silicon : 

SiO2  +  3  C  ->  SiC  +  2  CO. 

The  carborundum  is  vaporized  in  the  hottest  part  of  the  elec- 
tric furnace,  close  to  the  coke  core,  and  it  crystallizes  from 

the  state  of  vapor 
in  the  cooler  regions 
around  the  core. 
The  crystals  are  flat 
and  thin  and  often- 
times are  beautifully 
iridescent.  They 
usually  appear 
opaque  because  they 
contain  traces  of 
graphite,  but  the 
pure  substance  is 
colorless  and  transparent.  Carborundum  is  a  very  hard  sub- 
stance, —  harder  than  any  precious  stone  except  the  diamond. 
A  crystal  of  carborundum  will  scratch  glass  with  the  greatest 
ease,  but  carborundum  itself  is  easily  scratched  by  the 
diamond. 

291.    Calcium  Carbide  is  another  important  carbon  com- 
pound, which,  like  carborundum,  is  made  in  the  electric  fur- 


FIG.  59.  —  Section  of  Carborundum  Furnace. 


COMPOUNDS  WITH  NON-METALS  279 

nace.  The  raw  materials  are  lime  and  coke,  which  react 
according  to  the  equation  : 

CaO  +  3  C  ->-  CaC2  +  CO. 

The  calcium  carbide  thus  formed  is  a  hard,  dark  gray  solid. 
The  greatest  value  of  calcium  carbide  depends  upon  the  fact 
that  it  reacts  with  water  to  form  acetylene  gas  : 

CaC2  +  2  H2O^Ca(OH)2  +  C2H2. 


COMPOUNDS  WITH  NON-METALS 

292.  In  the  carbides,  carbon  plays  the  part  of  a  non- 
metallic    or    electro-negative    element.       When,    however, 
carbon  combines  with  the  more  active  non-metallic  elements, 
it  becomes  forced  to  take  up  the  electro-positive  role  itself 
and  thus  we  have  carbon  oxide  as  well  as  carbon  sulphide  and 
carbon  chloride. 

293.  Carbon  disulphide,  CS2,  is  made  by  the  direct  union 
of  carbon  and  sulphur  vapor  at  a  high  temperature.     It  is 
a  colorless  volatile  liquid  which  is  much  used  as  a  solvent 
in  rubber  cements.     It  also  dissolves  fats,  waxes,  iodine, 
sulphur,  and  many  other  substances  which  do  not  dissolve 
in  water.     Hence  its  greatest  use  is  as  a  solvent.     It  is  also 
used  to  exterminate  insect  pests  which  infest  grain,  and  to 
kill  ants.     It  should  be  used  with  caution  for  these  purposes, 
for  inhaling  its  vapor  is  injurious  and  may  even  prove  fatal. 

294.  Carbon  tetrachloride,  CCL,,  is  a  colorless  liquid  which, 
like  carbon  disulphide,  finds  its  principal  use  as  a  solvent. 
It  has  an  advantage  over  the  latter  in  that  it  is  non-inflam- 
mable, and  it  is  not  so  volatile.     It  is  usually  made  by 
treating  carbon  disulphide  with   chlorine, 

CS2  +  3  C12  ->  CC14  +  S2C12, 


280  COMPOUNDS  OF  CARBON 

and  distilling  it  off  from  the  resulting  mixture.  Large 
quantities  are  now  sold  under  various  fanciful  names  as 
a  cleansing  fluid  for  removing  grease  spots.  Being  non- 
inflammable,  it  is  safer  than  benzine  and  gasoline.  Other 
large  amounts  are  sold  for  use  in  fire  extinguishers.  These 
are  especially  efficient  in  putting  out  gasoline  fires.  The 
vapor  of  carbon  tetrachloride  is  not  only  non-inflammable, 
but  it  is  also  very  heavy  and  hangs  over  the  fire  and 
smothers  it. 

SUMMARY 

Free  carbon  exists  in  three  forms:  (1)  amorphous  carbon  (coke, 
charcoal,  lampblack,  gas  carbon,  coal)  ;  (2)  graphite ; 
(3)  diamond.  Amorphous  carbon  is  without  definite  form. 
Graphite  forms  hexagonal  crystals.  Diamond  crystallizes  in 
the  regular  system.  Graphite  is  one  of  the  softest  solid  sub- 
stances. Diamond  is  the  hardest  substance  known.  Graph- 
ite, diamond,  and  amorphous  carbon  all  burn  in  oxygen 
and  yield  only  carbon  dioxide.  The  differences  between 
amorphous  carbon,  graphite,  and  diamond  are  believed  to  be 
due  to  a  difference  in  molecular  structure. 

Carbon  dioxide  is  the  product  of  the  free  combustion  of  carbon ;  it  is 
stored  in  vast  quantities  in  nature  in  the  form  of  carbonates. 

Carbon  monoxide  is  a  valuable  fuel.  It  is  largely  present  in  water 
gas  and  in  producer  gas. 

Water  gas  is  made  by  blowing  steam  through  white-hot  coke. 

Producer  gas  is  made  by  blowing  air  through  a  deep  bed  of  hot  coal 
or  coke. 

Coal  gas  is  made  by  the  destructive  distillation  of  soft  coal  in  retorts. 

The  luminosity  of  flames  is  due  to  the  presence  in  them  of  heated 
solid  particles,  usually  of  carbon. 

Carbides :  Carbon  is  not  a  very  active  non-metal,  hence  the  carbides 
of  the  metals  are  not  as  easily  formed  as  the  oxides,  chlorides, 
and  sulphides. 

Iron  carbide  is  exceedingly  important  because  of  the  effect  of  its 
presence  upon  the  properties  of  iron  and  steel. 


QUESTIONS  281 

Silicon  carbide  (carborundum)  is  a  very  useful  artificial  abrasive. 
Calcium  carbide  is  important  as  a  source  of  acetylene. 
Carbon  disulphide  and  carbon  tetrachloride  are  compounds  of  carbon 
with  non-metals.    Their  chief  value  is  due  to  their  solvent 


powers. 


Questions 


1.  What  is  the  most  valuable  form  of  carbon?    What  is  the 
most  useful  form  ? 

2.  Of  what  substance  are  the  "  leads  "  of  lead  pencils  made? 
How  is  the  hardness  of  pencils  varied? 

3.  Why  is  graphite  used  on  bicycle  chains? 

4.  How  may  diamonds  be  made  artificially? 
6.    How  is  graphite  made  artificially? 

6.  Why  does  not  pure  water  gas  give  a  luminous  flame?     How 
may  it  be  made  to  do  so  ? 

7.  Explain  the  function  of  the  "  generator,"  the  "  carburetor" 
and  the  "  superheater  "  in  the  production  of  water  gas  as  shown  in 
Fig.  55.     Explain  the  operation  of  the  valves. 

8.  How  is  producer  gas  made?    Explain  the  diagram,  Fig.  56. 
What  is  the  service  performed  by  the  economizer?     What  is  the 
object  of  allowing  water  to  trickle  into  the  economizer  ? 

9.  How  is  coal  gas  made?     Explain  the  diagram,  Fig.  57. 

10.  Which  would  give  the  hotter  flame,  producer  gas  or  water 
gas?     Why? 

11.  How  is  carborundum  made? 

12.  How  is  calcium  carbide  made? 

13.  What  are  the  principal  uses  of  carbon  tetrachloride?    What 
advantage  has  it  over  carbon  disulphide? 


CHAPTER  XXIV 

COMPOUNDS    OF    CARBON     (Continued) 

295.  Organic  Chemistry.  The  compounds  of  carbon  arc 
studied  under  a  special  division  of  chemistry  known  as  or- 
ganic chemistry;  the  original  significance  of  the  name  was 
that  these  compounds  were  produced  by  the  life  processes 
of  plants  or  animals,  that  is,  of  living  organisms.  Some 
compounds,  it  is  true,  which  were  considered  organic  were 
formed  by  the  partial  decomposition  of  compounds  produced 
by  living  organisms  rather  than  formed  directly  by  the  or- 
ganisms themselves. 

As  examples  of  the  type  of  substances  formed  directly  in 
living  organisms,  we  have  starch  and  sugar.  Starch  is 
formed  in  potatoes,  in  grains,  and  in  many  other  kinds  of 
plants.  Sugar  is  formed  in  the  sugar  cane,  the  sugar  maple, 
and  the  sugar  beet.  Acetic  acid,  methyl  alcohol,  and  acetone 
are  examples  of  organic  substances  produced  indirectly  from 
others  which  were  formed  by  living  organisms.  These  three 
substances  result  when  wood  is  destructively  distilled.  Ben- 
zene, carbolic  acid,  naphthalene,  and  methane  are  organic 
substances  obtained  by  the  destructive  distillation  of  soft 
coal  (coal  being  the  product  of  the  partial  decomposition 
of  woody  material  which  grew  in  some  past  geologic  age). 
As  examples  of  organic  substances 'produced  by  the  growth  of 
animals,  we  have  the  various  protein  substances  such  as  egg 
albumin  (the  white  of  egg  is  mainly  albumin  and  water)  and 
gelatin  (which  is  extracted  from  bones  and  hides  by  boiling). 

282 


ORGANIC   CHEMISTRY  283 

Formerly  it  was  supposed  that  the  mysterious  agency  of 
life  was  essential  to  the  formation  of  organic  compounds, 
but  one  by  one  the  organic  compounds  have  been  synthesized 
in  the  laboratory  until  now  all  but  the  most  complicated  can 
be  built  up  by  strictly  inorganic  agencies  from  their  elements, 
these  elements  being  carbon  and  usually  hydrogen,  with  oxy- 
gen and  nitrogen  often,  and  sulphur,  phosphorus,  and  iron 
sometimes,  in  addition. 

Thus  there  is  really  no  hard  and  fast  line  between  organic 
and  inorganic  chemistry.  The  compounds  of  carbon  are  so 
numerous  and  the  chemistry  of  these  compounds  so  compli- 
cated that  it  is  more  convenient  to  separate  this  field 
of  chemistry  from  the  inorganic  field  for  the  purpose  of 
study. 

As  was  suggested  above,  it  was  once  thought  that  organic 
compounds  could  be  produced  only  by  the  agency  of  vital 
force.  To-day  it  is  recognized  that  chemical  reactions,  which 
are  in  every  way  analogous  to  the  reactions  which  take 
place  in  laboratory  test  tubes,  take  place  within  animal  and 
plant  tissues  and  that  the  life  and  growth  of  the  organisms 
depend  on  the  chemical  changes  rather  than  that  the  chemi- 
cal changes  depend  on  the  vital  force. 

In  our  study  of  inorganic  chemistry,  we  have  observed 
that  compounds  are  always  possible  between  metals  and  non- 
metals,  or,  in  other  words,  between  electro-positive  and  elec- 
tro-negative elements.  Now  carbon  is  neither  a  distinct 
metal  nor  a  distinct  non-metal,  and  in  its  ability  to  form  com- 
pounds it  shows  itself  markedly  different  from  other  elements. 
Its  valence  in  its  compounds  is  four,  and  the  most  striking 
feature  about  its  combining  relations  is  that  it  is  apparently 
a  matter  of  indifference  whether  these  four  valences  are  ex- 
erted in  holding  positive  or  negative  elements  or  whether 


284  COMPOUNDS  OF  CARBON 

they  are  divided,  part  holding  positive  and  the  rest  holding 
negative  elements.  Moreover,  carbon  possesses  a  peculiar 
ability  to  use  a  part  of  its  valences  in  uniting  with  other 
carbon,  while  at  the  same  time  the  rest  of  its  valences  are 
exerted  in  holding  other  elements.  On  this  account,  a  vastly 
increased  number  of  compounds  of  carbon  is  possible. 

Among  the  possible  compounds,  thousands  of  actual  ones 
are  known,  and  in  this  brief  chapter  on  organic  compounds  a 
few  only  of  the  more  interesting  of  the  carbon  compounds  can 
be  considered.  Compounds  consisting  only  of  carbon  and 
hydrogen  (called  hydrocarbons)  form  a  large  and  important 
group. 

METHANE  AND  THE  PARAFFIN  SERIES 

296.  Methane,  marsh  gas,  CH4,  is  the  simplest  member 
of  an  extended  series  of  organic  compounds.  The  gas  which 
arises  when  one  pokes  a  stick  into  the  mud  beneath  the  pools 
of  water  in  marshes  consists  mainly  of  this  substance.  In 
addition  to  being  formed  in  the  mud  of  marshes,  methane  is 
also  frequently  present  in  coal  mines,  resulting  from  the 
decomposition  of  coal,  and  its  presence  is  the  cause  of  many 
serious  explosions.  The  coal  miners  call  it  fire  damp.  It  is 
also  a  very  important  constituent  of  the  illuminating  gas 
that  is  obtained  by  the  destructive  distillation  of  coal,  and 
large  amounts  of  it  are  present  in  natural  gas. 

Carbon  tetrachloride  is  of  interest  in  its  relation  to 
methane.  In  methane  the  carbon  atom  has  attached  to  it 
four  positive  hydrogen  atoms,  whereas  in  carbon  tetrachlo- 
ride the  place  of  the  hydrogen  is  taken  by  four  electro-nega- 
tive chlorine  atoms. 

Compounds  with  the  formulas,  CH3C1,  CH2C12,  and  CHC13, 
are  also  known.  The  last  is  the  well-known  substance  chloro- 


THE  PARAFFIN  SERIES  285 

form.  In  these  compounds  carbon  is  united  with  both  posi- 
tive and  negative  elements.  Not  only  is  it  possible  to  re- 
place one  or  all  of  the  hydrogen  atoms  of  methane  with 
chlorine,  but  also  with  other  atoms  or  groups  of  atoms. 

297.  The  Paraffin  Series.  A  compound  is  known,  which 
may  be  regarded  as  methane  in  which  one  hydrogen  atom  has 
been  replaced  by  the  group  CH3.  This  new  compound  is 
called  ethane.  Here,  carbon  may  be  considered  as  united  with 
itself  as  well  as  with  an  electro-positive  element,  hydrogen. 

H  H       H 

I                                   I         I 
H— C— H  H— C C— H 

I  ! 

H  H       H 

Methane  Ethane 

Ethane,  like  methane,  is  found  in  illuminating  gas  made 
by  the  destructive  distillation  of  soft  coal. 

By  displacing  an  atom  of  hydrogen  from  ethane  by  means 
of  the  CH3  group,  a  new  compound  known  as  propane  results. 

H    H    H 

I      I      I 
H— C— C— C— H 

I      I      I 
H    H    H 

Propane 

By  replacing  an  atom  of  hydrogen  of  propane  by  a 
group,  butane  is  obtained. 

H    H    H    H 

I       I       I       I 
H— C— C— C— C— H 

III! 

H    H    H    H 

Butane 


286  COMPOUNDS  OF  CARBON 

Butane  is  one  of  the  more  volatile  components  of  petroleum. 
Petroleum,  which  is  a  crude,  offensive  oil  drawn  from  the 
earth  in  various  localities,  is  in  the  main  a  mixture  of  many 
hydrocarbon  compounds  belonging  to  the  series  whose  first  few 
members  have  just  been  mentioned.  In  this  series  with  the 
introduction  of  each  new  CH2  group,  the  molecular  weight, 
of  course,  increases,  and  it  is  interesting  to  note  that  the 
boiling  points  of  the  successive  substances  increase  in  a  some- 
what regular  fashion.  Other  properties,  such  as  specific 
gravity,  for  example,  also  vary  with  varying  molecular  weight. 
The  higher  the  molecular  weight,  the  denser  the  substance 
becomes. 

298.  Distillation  of  Petroleum.  In  separating  the  great 
mixture  of  hydrocarbons,  known  as  crude  petroleum,  advan- 
tage is  taken  of  the  variation  in  boiling  points,  and  by  distill- 
ing off  fractions  of  the  whole  into  separate  containers,  the 
crude  mixture  is  separated  into  portions  of  varying  volatility, 
the  most  volatile  portions  being  first  to  come  off.  Petro- 
leum contains  so  many  different  members  of  the  paraffin 
series,  and  the  molecular  weights  and  hence  the  boiling  points 
are  so  near  to  each  other,  that  it  is  not  practically  possible  by 
distillation  to  obtain  one  of  the  pure  substances  entirely  by 
itself. 

The  first  fractions  containing  several  of  the  more  vol- 
atile members  of  the  paraffin  series  are  condensed  and 
sold  under  the  trade  names  of  cymogene  and  rhigolene. 
These  are  very  volatile  liquids,  and  they  are  sometimes  used 
to  chill  portions  of  the  body  surface  to  deaden  sensation  dur- 
ing minor  surgical  operations.  The  rapid  evaporation  of  the 
very  volatile  liquid  takes  up  the  heat  of  the  part  sprayed 
with  it,  thus  chilling  the  flesh  and  causing  it  to  become 
numb.  The  later  and  less  volatile  fractions  -from  petro- 


UNSATURATED  HYDROCARBONS         287 

leum  are  called  successively  gasoline,  naphtha,  benzine  (not 
benzene)  and  kerosene. 

After  the  kerosene  is  distilled  off,  there  remains  still  a 
mixture  of  many  of  the  higher  hydrocarbons  of  the 
paraffin  series  from  which  heavier  oils  are  distilled.  When 
these  oils  are  chilled,  flakes  of  paraffin  separate.  This  is 
removed  by  filter  pressing  and  the  oil  is  used  for  lubri- 
cants. The  viscous  substance  known  as  vaseline  is  ob- 
tained from  the  residue  left  when  the  crude  oil  is  distilled 
in  a  vacuum. 

The  last  volatile  fractions  of  the  petroleum  come  off  only 
when  the  bottom  of  the  still  has  risen  to  a  red  heat;  and 
the  residue  then  left  in  the  still  is  coke  in  a  form  which  is 
highly  valued  for  making  electric  light  carbons. 

All  of  these  substances  obtained  from  petroleum  are  very 
resistant  to  attack  by  chemicals  except  at  high  tempera- 
tures. Hence  the  origin  of  the  name  of  the  series,  paraffin, 
suggesting  lack  of  affinity. 

UNSATURATED  HYDROCARBONS 

299.  A  number  of  other  natural  series  of  hydrocarbons  are 
known,  the  ethylene  series  in  which  the  first  member  is 
ethylene,  C2H4,  and  the  acetylene  series  in  which  acetylene, 
C2H2,  is  the  first  member,  being  among  the  more  important. 
The  relation  between  molecular  weights  and  properties  which 
was  noted  in  the  paraffin  series  is  found  also  in  the  other 
series  of  hydrocarbons. 

In  ethylene  and  acetylene  it  is  believed  that  carbon  still 
possesses  its  universal  valence  of  four,  but  in  order  to  main- 
tain this  belief,  it  becomes  necessary  to  suppose  that  more  than 
a  single  valence  of  one  carbon  atom  is  employed  in  holding 

B.   AND  W.  CHEM. 19 


288  COMPOUNDS  OF  CARBON 

an  adjacent  carbon  atom.  Thus  ethylene  is  supposed  to 
possess  a  double  bond  jj\  /H 

>C=C/ 
W  XH 

and  acetylene  a  triple  bond 

H—  C  =  C—  H 

The  justification  of  the  term  unsaturated  in  describing  com- 
pounds of  this  character  lies  in  the  fact  that  they  are  literally 
unsaturated.  For  example,  ethylene,  when  mixed  with 
hydrogen  and  passed  through  a  heated  tube  containing  a 
suitable  catalyzer,  takes  up  additional  hydrogen  easily  and 
is  changed  into  ethane,  a  saturated  compound 

ri2vy  =—  (_yii2    i    -H-2   —  ^    .tiaO  -  Oils 

or,  treated  with  chlorine,  ethylene  readily  changes  into  di- 
chlor-ethane. 

H  H  Ck  Cl 


C=C         +  C12  —  ->-    l 
W  XH  H/  \H 

As  already  indicated  in  the  preceding  chapter,  ethylene 
is  an  important  constituent  of  illuminating  gas,  and  acetylene 
finds  extensive  use  as  an  illuminant,  as  in  automobile  head- 
lights. 

CARBOHYDRATES 

300.  There  is  another  great  class  of  carbon  compounds, 
known  as  carbohydrates,  in  which  hydrogen  and  oxygen  in 
precisely  the  right  proportion  to  form  water  are  united  with 
carbon.  Starch,  sugars,  gums,  and  cellulose  are  carbohy- 
drates. The  formulas  of  a  few  of  the  many  carbohydrates 
are  as  follows  : 


Starch  Grape  sugar  Cane  sugar  Cellulose 


SUGARS  289 

It  is  seen  that  starch  and  cellulose  have  the  same  formula 
with  the  exception  of  the  subscripts  ra  and  n.  These  sub- 
scripts m  and  n  signify  some  definite  but  unknown  numbers, 
probably  large  and  probably  different. 

301.  Sugars.  Sugars  are  found  in  nature  in  the  sap  of 
plants  and  the  juices  of  fruits.  The  sugars  are  built  in  the 
leaves  of  plants  by  the  interaction  of  the  carbon  dioxide  of 
the  air  and  the  water  of  the  sap  under  the  influence  of  sun- 
light and  the  chlorophyll,  or  green  coloring  matter  of  the 
leaves.  The  energy  of  the  sunlight  is  consumed  in  splitting 
up  the  carbon  dioxide  so  that  oxygen  is  returned  to  the  air, 
while  the  carbon  and  water  unite,  forming  carbohydrates. 

The  sugars,  which  are  soluble  in  water,  are  conveyed  by  the 
sap  to  all  parts  of  the  plant  and  are  converted  into  starch, 
or  cellulose,  or  other  sugars  by  the  chemical  addition  or 
subtraction  of  water  as  the  needs  of  the  plant  may  require. 

Cane  sugar  is  the  variety  of  sugar  found  in  the  sap  of  the 
sugar  cane,  the  sugar  beet,  and  the  maple  tree. 

Grape  sugar  is  found  in  grapes  and  other  fruits.  It  is  less 
sweet  than  cane  sugar.  Starch  can  be  converted  artificially 
into  grape  sugar  by  boiling  with  dilute  acids,  this  treatment 
promoting  the  addition  of  water  according  to  the  equation 

(C6Hi0O5)n  +  n  H2O  ->  n  C6Hi2O6 

Starch  Grape  sugar 

Enormous  quantities  of  corn  starch  are  thus  treated  in  mak- 
ing commercial  glucose,  a  product  which  consists  mainly  of 
grape  sugar  and  dextrin  (see  below) ,  the  latter  being  an  inter- 
mediate product  between  starch  and  grape  sugar.  Commer- 
cial glucose  is  ordinarily  a  thick  sirupy  product  and  is  often 
sold  for  table  use  under  the  name  of  corn  sirup.  Evapo- 
rated to  dryness,  it  yields  solid  "  grape  sugar." 


290  COMPOUNDS  OF  CARBON 

302.  Starch  is  insoluble  in  water  and  it  is  the  form  in  which 
carbohydrates  are  stored  up  by  plants  for  their  future  use  as 
foods.     It  is  thus  found  especially  in  bulbs,  tubers,  and  seeds, 
where  it  is  accumulated    for  the    nutrition  of    the  young 
plants. 

Potatoes  are  composed  almost  wholly  of  starch  and  water. 
Wheat,  rice,  barley,  corn,  and,  in  fact,  all  grains  are  composed 
largely  of  starch.  Consequently,  the  food  value  to  man  as 
well  as  to  the  plants  themselves,  of  all  kinds  of  grains,  tubers, 
bulbs,  etc.,  is  largely  due  to  their  content  of  starch. 

Starch  before  it  is  assimilated  in  the  human  body  is  con- 
verted back  to  soluble  sugars  by  the  action  of  the  digestive 
juices. 

Starch  is  always  found  in  plants  in  the  form  of  small  com- 
pact granules,  and  it  is  usually  possible  to  determine  the 
source  of  any  particular  starch  by  a  microscopic  examina- 
tion of  the  starch  granules. 

Dextrin,  which  is  the  adhesive  substance  with  which  the 
backs  of  postage  stamps  are  coated,  is  an  intermediate  prod- 
uct between  starch  and  sugar,  and  it  is  obtained  by  heating 
starch.  Dextrin  is  formed  to  some  extent  in  baking  bread, 
and  it  is  present  in  the  browned  crust. 

303.  Cellulose  is  the  chemical  substance  of  which  the 
fibrous  and  woody  part  of  plants  is  composed.     It  is  not 
used  as  food  by  man  (except  in  small  quantities  in  the  tender 
green  leaves  of  lettuce,  spinach,  etc.),  but  it  finds  enormous 
use  in  the  textile  industries  and  in  paper  making.     Cotton, 
linen,  and  wood  fiber,  for  example,  are  almost  pure  cellulose. 

Wood  consists  of  bundles  of  cellulose  fibers  cemented 
together  with  resinous  or  gummy  material.  Although  old 
linen  rags  furnish  the  cellulose  fiber  for  some  of  the  strongest 
grades  of  paper,  the  principal  source  of  the  cellulose  is  wood. 


ALCOHOLS  291 

The  fibers  are  sometimes  torn  apart  mechanically  for  the 
cheapest  paper,  but  usually  wood  chips  are  treated  chemically 
to  dissolve  out  the  binding  material  and  thus  leave  the  pure 
cellulose  as  a  loose  mass  of  fibers. 

The  paper  used  in  the  laboratory  for  making  filters  is 
practically  pure  cellulose.  Other  grades  of  paper  are  likely 
to  be  filled  with  mineral  matter  to  give  weight,  or  with  glue 
or  casein  to  give  a  hard,  smooth  finish. 

ALCOHOLS 

304.  Alcohol,  C2H5OH,  is  prepared  by  the  action  of  yeast, 
a  microscopic  plant  cell,  upon  grape  sugar,  or  other  type  of 
sugar.  These  little  plants  consume  a  great  deal  of  sugar 
in  their  life  process,  and  they  increase  in  numbers  with  as- 
tonishing rapidity  when  placed  in  a  favorable  medium.  The 
reaction  brought  about  by  the  yeast  plants  is  approximately 
expressed  by  the  equation 

C6Hi2O6  -+  2  CO2  +  2  C2H5OH. 

Heat  is  evolved  in  this  reaction,  and  carbon  dioxide  is  formed 
as  well  as  alcohol.  Since  most  of  the  sugar  thus  fermented 
in  the  commercial  preparation  of  alcohol  is  derived  from  the 
starch  of  grains,  this  kind  of  alcohol  is  known  as  grain  alco- 
hol, to  distinguish  it  from  other  alcohols. 

When,  in  the  fermentation  of  the  grain,  the  alcohol  reaches 
a  concentration  of  10  to  12  per  cent,  the  action  becomes 
very  slow  and  almost  ceases.  By  fractional  distillation, 
the  alcohol  is  now  separated  from  the  water  and  the  solid 
residue,  so  that  a  95  per  cent  alcohol  is  obtained  in  the  dis- 
tillate. 

The  distillation  of  alcohol  is  an  important  industry,  since 
alcohol  finds  many  uses  in  the  arts,  —  as  a  fuel,  as  a  solvent, 


292  COMPOUNDS  OF  CARBON 

and  in  many  other  ways.  Large  quantities  of  it  are  con- 
sumed in  a  more  or  less  diluted  condition  in  spirituous  and 
fermented  liquors. 

From  the  standpoint  of  chemistry  as  well  as  from  that  of 
economics,  the  use  of  alcohol  in  beverages  is  wasteful.  In 
fermenting  the  sugars  much  energy  is  lost  as  heat,  and  much 
labor  that  might  be  used  productively  is  required  to  care  for 
the  process.  The  distillation,  also,  requires  much  heat, 
energy,  and  labor,  and  the  products  are  stored  for  long 
periods,  thus  keeping  much  capital  idle. 

From  the  physiological  standpoint  alcohol  is  a  powerful 
poison.  Although  it  has  been  shown  by  scientists  that 
alcohol  is  burned  in  the  body  and  furnishes  heat  and  energy 
like  the  carbohydrates,  nevertheless  alcohol  has  very  harmful 
effects  upon  the  nervous  system  and  through  that  upon 
the  processes  of  assimilation  of  food.  Pure  alcohol  is  a 
deadly  poison;  somewhat  diluted,  as  in  whiskey,  rum,  and 
brandy,  it  is  very  harmful;  and  more  highly  diluted,  as  in 
fermented  beverages,  it  is  injurious  and  its  use  is  not  advisable 

305.  Denatured  Alcohol.  On  account  of  the  far-reaching 
evil  effects  of  the  use  of  alcohol  as  a  beverage  governments 
have  frequently  placed  heavy  internal  revenue  taxes  upon 
the  traffic  in  alcohol,  thus  limiting  its  use  and  at  the  same 
time  raising  revenue  for  governmental  expenses. 

The  resulting  high  price  of  alcohol  has  in  times  past  limited 
its  use  as  a  fuel  and  as  a  solvent  in  the  arts  and  industries. 
Recently  the  United  States  government  has  permitted 
the  sale  without  tax  of  alcohol  which  has  been  "  denatured," 
as  it  is  called,  by  the  addition  of  certain  other  substances. 
The  object  of  this  addition  is  to  render  the  alcohol  unfit 
for  use  as  a  beverage  without  in  any  way  harming  it  for 
industrial  purposes.  Various  substances,  among  them  pyri- 


ORGANIC  ACIDS  293 

dine,  may  b'e  used  to  denature  alcohol.  A  substance  with 
an  evil  odor  is  usually  employed  so  as  to  render  the  mix- 
ture offensive  to  drink. 

306.  Wood  Alcohol,  CH3OH,  is  obtained  by  the  destruc- 
tive distillation  of  wood.     Wood  alcohol  is  very  similar  to 
grain  alcohol  in  its  properties  and  these  two  alcohols  can 
be  used  almost  interchangeably  for  fuel  and  solvent  pur- 
poses.    It  should  never  be  used  as  a  constituent  of  beverages, 
because,  although  its  poisonous  properties  are  of  the  same 
general  character  as  those  of  grain  alcohol,  it  is  a  so  much 
more  violent  poison  that  its  use  is  likely  to  prove  imme- 
diately fatal.     The  vapor  even  is  dangerous  if  inhaled  in 
quantity,  and  its  action  upon  the  eyes  is  likely  to  cause 
blindness. 

ORGANIC  ACIDS 

307.  Organic   acids   are   found   in  many  fruits,   notably 
citric  acid  in  the  lemon,  orange,  and  other  citrus  fruits,  malic 
acid  in  the  apple,  and  the  acid  salt  of  tartaric  acid  (cream 
of  tartar)  in  grapes.     In  addition  to  these  natural  fruit  acids 
there  are  a  great  many  acids  which  are  formed  by  the  partial 
oxidation  of  organic  substances  like  the  alcohols  and  sugars. 
Organic  acids  are,  as  a  rule,  very  weak  as  compared  with  the 
mineral  acids,  hydrochloric,  nitric,  and  sulphuric.     Like  the 
carbohydrates,  they  are  oxidizable  in  the  body ;  hence  their 
desirability  in  foods    to    which   they   impart    a   mild    and 
pleasing  sour  taste. 

308.  Acetic  acid,  H(C2H3O2),  perhaps  ranks  first  in  im- 
portance among  the  organic  acids.     It  is  the  acid  of  vine- 
gar.    Cider  is  the  product  of  the  alcoholic  fermentation  of 
apple  juice.     By  allowing  cider  or  other  fermented  liquor 
of   similar  alcohol  content  to  undergo  a  different  sort  of 


294  COMPOUNDS  OF  CARBON 

fermentation  under  the  influence  of  certain  bacteria,  in  the 
presence  of  air,  an  oxidation  of  the  alcohol  to  acetic  acid 
takes  place. 

H    H  HO 

I       I  II 

H— C— C— O— H  +  O2  -^  H—  C— C— O— H  +  H2O. 

i  I 

H    H  H 

Alcohol  Acetic  acid 

For  industrial  uses,  acetic  acid  is  made  by  the  destructive 
distillation  of  wood.  The  cellulose  is  not  completely  decom- 
posed by  the  heat,  but  it  is  broken  down  into  a  great  variety 
of  volatile  organic  substances,  which  distill  off,  and  charcoal, 
which  remains  in  the  retort.  Wood  alcohol,  as  already  men- 
tioned, is  one  important  product  of  the  distillation ;  acetic 
acid  is  another. 

It  should  be  noted  that  although  practically  all  organic 
compounds  contain  hydrogen,  not  all  are  acids.  Even  in 
acetic  acid,  only  one  of  the  hydrogens  functions  as  acid 
hydrogen.  Hence  in  the  formula,  H(C2H3O2),  a  single  hy- 
drogen is  placed  alone,  whereas  the  other  three  are  placed 
with  the  acid  radical  (C2H3O2). 

FATS 

309.  Fats  are  compounds  of  carbon,  hydrogen,  and  oxygen, 
and  they  may  be  regarded  as  a  product  of  the  union  of  cer- 
tain higher  alcohols  with  certain  higher  acids,  called  fatty 
acids,  much  as  salts  are  the  product  of  the  union  of  bases 
and  acids.  Instead  of  acting  instantly  with  acids,  as  do 
bases,  the  alcohols  react  very  slowly. 

Stearin  is  the  principal  fat  in  tallow  and  is  represented 
by  the  formula  C3H5(Ci8H35O2)3.  It  is  derived  from  glycerol, 


SOAPS  295 

C3H5(OII)3,  an  alcohol  with  three  hydroxyl  groups,  and 
stearic  acid,  H(Ci8H35O2).  Three  molecules  of  stearic  acid 
are  necessary  to  unite  with  glycerol  according  to  the  equation 

C3H5(OH)3+  3  H(C18H3502)  ->  C3H5(C18H35O2)3  +  3  H2O. 

Glycerol  Stearic  acid  Stearin 

The  equation  is  similar  to  that  of  a  neutralization  reaction, 
but  it  should  be  remembered  that  the  glycerol  is  not  per- 
ceptibly a  base  and  such  a  reaction  can  be  carried  out  only 
with  difficulty  in  the  laboratory.  Fats  are  formed  mainly 
in  the  slow  chemical  processes  taking  place  in  animal  and 
plant  tissues. 

The  reaction  represented  by  the  reverse  of  the  above 
equation  can  take  place  rather  easily,  and  fats  on  standing 
often  become  rancid,  due  in  large  part  to  the  separation  in 
this  manner  of  the  free  fatty  acid.  This  action  is  known  as 
hydrolysis. 

Fats  are  valuable  foods;  they  yield  nearly  two  and  one 
quarter  times  as  much  heat  when  oxidized  in  the  body  as 
carbohydrates  or  proteins.  They  are  thus  splendid  food  for 
cold  weather  or  cold  climates,  but  they  are  somewhat  difficult 
of  digestion.  Broadly  speaking,  all  fats  and  oils  obtained 
from  animals  and  plants,  including  butter,  lard,  olive  oil, 
cottonseed  oil,  and  castor  oil,  are  similar  chemically  to 
stearin. 

310.  Soaps  are  made  by  the  action  of  strong  mineral 
bases  upon  fats.  For  example,  when  sodium  hydroxide 
acts  upon  stearin,  glycerol  is  separated  from  the  fat  and 
sodium  stearate,  a  salt  of  the  organic  acid,  is  formed. 

3  NaOH  +  C3H5(C18  H35O2)3  ->  C3H5(OH)3  +  3  Na(C18  H35O2) . 

Stearin  Glycerol  Soap 

Sodium  stearate  is  a  typical  soap. 


296  COMPOUNDS  OF  CARBON 

PROTEINS 

311.  The  albumin  of  the  white  of  eggs  is  one  of  a  number 
of  carbon  compounds  which  may  be  classed  together  under 
the  name  of  proteins.  The  proteins  contain  carbon,  hydro- 
gen, oxygen,  and  nitrogen  always,  and  they  may  contain 
small  amounts  of  other  elements,  as  phosphorus,  sulphur,  and 
iron.  To  this  class  of  carbon  compounds  belong  the  tissue- 
building  foods  such  as  are  found  in  greatest  abundance  in 
meat,  milk,  eggs,  cheese,  the  gluten  of  wheat,  beans,  and  peas. 
It  was  said  in  Chapter  VII  (while  discussing  the  fixation  of 
atmospheric  nitrogen)  that  legumes  were  able,  by  the  aid 
of  bacteria  living  on  their  roots,  to  cause  the  free  nitrogen 
of  the  air  to  combine  with  other  elements  to  form  nitrogenous 
compounds.  These  compounds  become  stored  up  as  pro- 
teins principally  in  the  seeds  of  the  leguminous  plants.  The 
animal  foods  —  meat,  milk,  eggs,  and  cheese  —  contain  pro- 
teins derived  from  the  plant  food  eaten  by  the  animals. 
Animals  possess  the  power  to  make  over  many  protein  sub- 
stances into  other  proteins  which  are  better  adapted  to  the 
needs  of  the  particular  organism,  but  animals  do  not  them- 
selves seem  to  be  able  to  put  together  the  elementary  sub- 
stances to  form  the  proteins. 

SUMMARY 

Many  of  the  compounds  of  carbon,  which  it  was  formerly  supposed 
could  only  be  formed  in  living  organisms,  can  to-day  be  pre- 
pared in  the  laboratory. 

The  valence  of  carbon  is  four.  These  valences  may  be  exercised  in 
holding  electro-positive  or  electro-negative  elements  or  part  of 
both  at  the  same  time.  Some  of  the  carbon  valences  may 
also  hold  other  carbon  atoms.  Many  thousands  of  different 
compounds  of  carbon  are  known. 

Methane  is  the  first  member  of  a  series  of  hydrocarbons  called  the 
paraffin  series.  Ethane,  propane,  and  butane  are  the  next  three 


SUMMARY  297 

members  of  this  series.  Methane  is  one  of  the  products 
formed  when  organic  substances  decompose  without  a  free  sup- 
ply of  air.  It  is  found  in  coal  mines,  and  the  gas  from  marshes 
is  mainly  methane.  Natural  gas  also  contains  methane. 

Carbon  tetrachloride  is  a  useful  solvent.  Its  composition  shows 
that  it  may  be  regarded  as  a  derivative  of  methane.  In  it  the 
carbon  is  united  to  an  electro-negative  element. 

The  paraffin  series  illustrates  the  remarkable  relation  between 
molecular  weights  and  properties  in  an  homologous  series. 

Petroleum  oil  consists  mainly  of  various  members  of  the  paraffin 
series.  It  may  be  roughly  separated  by  fractional  distilla- 
tion into  the  various  commercial  products  of  petroleum. 

Ethylene  and  acetylene  are  the  first  members  of  two  unsaturated 
series  of  hydrocarbons. 

Carbohydrates  are  a  very  important  class  of  carbon  compounds. 
Many  of  our  most  valuable  foods  are  carbohydrates,  as,  for 
example,  sugar  and  starch.  Dextrin  and  cellulose  also  are 
carbohydrates. 

Alcohol  is  another  carbon  compound  of  considerable  use  in  the  arts. 
It  is  also  used  in  many  beverages,  but  such  use  is  both  uneco- 
nomical and  unwise  from  the  standpoint  of  health.  Wood 
alcohol  is  chemically  similar  to  grain  alcohol.  It  is  far  more 
poisonous. 

Organic  acids  are  found  in  many  fruits.  They  are  far  weaker  than 
the  mineral  acids  H2S04,  HC1,  and  HNOs.  Some  organic 
acids  are  derived  from  the  oxidation  of  alcohols  or  sugars  by 
fermentation.  Acetic  acid,  the  acid  of  vinegar,  is  the  princi- 
pal one  of  these.  Acetic  acid  is  also  obtained  by  the  destruc- 
tive distillation  of  wood. 

Fats  are  valuable  foods.  They  are  formed  from  alcohols  and 
organic  acids.  They  may  hydrolyze,  whereby  the  fatty  acid 
is  set  free.  Soap  is  the  sodium  salt  of  some  of  the  fatty  acids 
of  common  fats.  Glycerol  is  a  by-product  of  the  soap  in- 
dustry. 

The  proteins  are  organic  compounds  containing  nitrogen,  and  they 
are  valuable  foods.  Plants  form  proteins  directly  from  the 
simpler  substances,  but  animals  only  remake  proteins  from 
other  proteins  obtained  from  plants. 


298  COMPOUNDS  OF  CARBON 

Questions 

1.  Why  is  organic  chemistry  usually  studied  apart  from  inor- 
ganic chemistry? 

2.  Is  there  any  real  difference  between  the  two  branches  of 
chemistry  ? 

3.  What  peculiarity  in  combining  ability  distinguishes  carbon 
from  other  elements  ? 

4.  What  has  this  peculiarity  to  do  with  the  existence  of  a  vast 
number  of  organic  compounds? 

5.  Any  member  of  the  paraffin  series  may  be  represented,  by 
the  general  formula  CnH(2n  +  2).     What,  then,  is  the  formula  of /that 
member  of  the  series  which  has   18  carbon  atoms?     10  carbon 
atoms?     5  carbon  atoms? 

6.  How  is  gasoline  obtained  from  crude  petroleum? 

7.  What  peculiarity  of  composition  distinguishes  the  carbohy- 
drates? 

8.  What  is  grape  sugar  commonly  called  ? 

9.  What  weight  of  alcohol  might  be  obtained  by  the  complete 
transformation  of  100  grams  of  glucose  ?    (See  equation  on  page  291 .) 

10.   Why  is  the  use  of  alcohol  as  a  beverage  uneconomical,  aside 
from  its  harmful  effect  ? 


CHAPTER  XXV 

THE   IONIC    THEORY 

312.  Electrical   Conditions   of   Acid   Hydrogen.     It   was 
shown  in  Chapter  XIX  that  the  characteristic  properties  of 
acids  must  be  attributed  to  the  presence  in  them  of  hydro- 
gen which  is  in  a  different  state  of  combination  from  ordi- 
nary combined  hydrogen  and  which  has  been  designated  as 
dizolaceable  or  acid  hydrogen. 

The  acid  properties  of  the  hydrogen  in  acids  are  not  mani- 
fest when  the  acid  is  pure  and  dry,  but  are  developed  when 
the  acid  is  dissolved  in  water.  It  is  now  generally  believed 
that  the  solvent  in  some  way  separates  the  molecules  of  acid 
into  two  components,  of  which  one  is  the  hydrogen  atom  (or 
atoms  in  acids  like  H2SO4  and  H3PO4)  and  the  other  is  the  acid 
radical.  Furthermore,  it  is  believed  that  the  hydrogen  atom 
so  separated  is  charged  with  positive  electricity  and  the  acid 
radical  is  charged  with  negative  electricity.  These  electri- 
cally charged  components  of  the  acid  are  known  as  ions. 
According  to  this  supposition,  the  hydrogen  ion  is  a  compo- 
nent of  every  acid  solution,  and  it  is  the  one  which  produces 
the  acid  properties. 

313.  Electrostatic  Attraction  and  Repulsion.     It  is  a  well- 
known  fact  that  when  the  hair  is  combed  on  a  cold,  dry  day 
the  hair  stands  on  end  as  if  the  separate  hairs  repelled  each 
other,  but  that  the  hairs  are  all  attracted  to  the  comb.     The 
friction  of  the  comb  on  hair  leaves  the  latter  charged  with 

299 


300  THE  IONIC   THEORY 

positive  electricity  and  the  former  with  negative  electricity. 
/    This  simple  experiment  illustrates  a  general  law  of  electricity, 
y^     that  like  charges  repel  and  unlike  charges  attract  each  other. 
The  electricity  that  is  available  in  the  laboratory  for  chemi- 
cal work  is  generated  either  chemically  in  electro-chemical 
cells,  or  mechanically  by  dynamos  in  which  coils  of  wire  are 
rotated  in  magnetic  fields,  but  scarcely  ever  by  friction. 

314.  Electricity  is  something  very  intangible.  We  do 
not  feel  like  calling  it  a  substance,  for  it  does  not  seem  to  have 
weight.  It  probably  pervades  all  ordinary  substances,  and 
yet  its  presence  is  commonly  unnoticed.  We  are  not  ordi- 
narily conscious  on  a  windless  day  that  we  are  living  at  the 
bottom  of  a  sea  of  air  which  presses  heavily  upon  us  and  every- 
thing around  us.  But  we  become  conscious  of  the  presence 
of  the  air  when  it  is  in  motion,  as  when  the  wind  blows,  or 
when  it  is  compressed  as  in  an  automobile  tire,  or  when  it  is 
rarefied,  as  in  an  electric  light  bulb,  which  collapses  with  a  loud 
report  when  it  is  broken.  In  these  respects,  electricity  is 
not  very  unlike  air.  Friction  may  be  supposed  to  act  in  a 
measure  like  an  air  pump  and  to  withdraw  electricity  from 
one  object  and  compress  it  into  another.  If  positive  elec- 
tricity is  at  a  higher  pressure  in  the  hair  than  in  the  surround- 
ings, it  makes  itself  obvious,  and  we  see  the  hair  stand  on  end 
as  a  result.  Similarly,  if  negative  electricity  is  at  a  higher 
pressure  on  the  rubber  comb  than  in  surrounding  objects,  it 
again  makes  that  fact  obvious  by  its  ability  to  attract  the 
positively  electrified  hair.  Air  can  be  forced  by  a  pump 
through  a  pipe  and  at  the  end  of  the  pipe  be  made  to  do  work, 
as  in  running  a  compressed  air  drill.  Likewise,  electricity 
may  be  forced  by  an  electric  pump,  —  that  is,  a  dynamo, 
or  an  electro-chemical  cell,  —  to  flow  through  a  circuit  and 
to  do  work  as  in  running  a  motor  or  in  decomposing  a 


NON-CONDUCTORS 


301 


chemical  compound  like  sodium  chloride  into  its  constituent 
elements. 

315.  Metallic   Conduction.     It  is  a  matter  of  common 
knowledge  that  metals  are  invariably  used  to  conduct  elec- 
tric currents,  and  that  non-metallic  substances,  such  as  glass, 
rubber,   and   sulphur,   are   used   as   insulators.     Electricity 
passes  with  great  ease  through  metals,  in  fact,  its  flow  through 
metals  can  be  likened  to  the  flow  of  water  through  pipes. 
So  far  as  we  can  detect, 

the  metal  suffers  no  more 
alteration  by  the  passage 
of  the  current  than  the 
pipe  suffers  by  the  pas- 
sage of  water. 

316.  Non-Conductors. 
Substances    other    than 
metals    ordinarily    offer 
great   resistance   to    the 
passage     of    electricity, 
arid  on  this  account  are 
termed    non-conductors. 
Non-metallic  substances 
include     not     only    the 
non-metallic       elements 
themselves,  but  also  com- 
pounds, even  compounds 

Containing    metals.       For         FIG.  60.  —  Apparatus  for  showing  Electrical 
i  j«  11      «i        Conductivity.    The  brilliancy  with  which  the 

example,  sodium  chloride    lamp  glows  is  a  measure  of  the  conductivity 

possesses     none     of     the     °f  ^ne  substance  placed  between  the  elec- 
.     .  trodes. 

characteristics  ot  metals 

and  when  it  is  dry  it  does  not  conduct  electricity.     Water 

is  another  distinctly  non-metallic  substance  and,  like  the 


302  THE  IONIC  THEORY 

other  substances  in  its  class,  it  is  when  pure  an  almost  com- 
plete non-conductor  of  electricity. 

317.  Electrolytic  Conduction.  We  now  come  to  a  most 
remarkable  series  of  phenomena.  Water  as  stated  does  not 
conduct  electricity  when  pure,  no  more  does  dry  sodium 
chloride ;  but  dissolve  sodium  chloride  in  water,  and  a  solu- 
tion is  obtained  which  conducts  the  current  readily. 

This  may  be  strikingly  shown  if  an  electric  circuit  is  ar- 
ranged as  shown  in  Fig.  60.  Connections  are  made  at 
the  points  marked  +  and  —  with  the  two  wires  of  the  110- 
volt  direct-current  incandescent  lighting  system.  Alternat- 
ing current  will  serve  equally  well.  Between  these  two  con- 
nections there  are  inserted  in  the  circuit  an  incandescent 
lamp  and  a  gap  between  two  platinum  wires  (electrodes). 
These  wires  are  suspended  from  binding  clamps,  parallel  and 
near  to  each  other,  but  not  touching.  The  circuit  as  now 
arranged  passes  through  the  incandescent  lamp  but  is  broken 
by  the  gap  between  the  platinum  electrodes.  If  these  are 
connected  by  a  piece  of  metal,  the  circuit  is  completed  and 
the  lamp  glows  with  its  full  brilliancy.  If  a  piece  of  non- 
conducting substance  is  held  so  as  to  touch  both  electrodes, 
no  current  passes  and  the  lamp  does  not  glow. 

On  holding  a  dry  lump  of  common  salt  so  that  it  touches 
both  platinum  electrodes,  the  lamp  does  not  glow  and  the  salt 
is  thus  shown  to  be  a  non-conductor.  On  raising  a  small 
vessel  filled  with  distilled  water  from  underneath  so  that  both 
electrodes  dip  in  the  liquid,  no  light  is  seen  in  the  lamp,  and 
water  is  also  shown  to  be  a  non-conductor.  If,  nt>w,  a  little 
of  the  salt  is  dissolved  in  the  water  and  the  conductivity  of 
the  solution  is  tested  in  the  same  way,  the  lamp  is  seen  to 
glow  brilliantly,  and  the  solution  of  sodium  chloride  is  proved 
to  be  a  good  conductor. 


ELECTROLYTIC   CONDUCTION  .     303 

• 
318.    Not  only  does  a  solution  of  sodium  chloride  conduct 

strongly,  but  solutions  of  all  salts  conduct  in  about  the  same 
degree.  Hydrochloric  and  sulphuric  acids,  which  we  have 
spoken  of  as  strong  acids,  likewise  cause  the  lamp  to  glow 
brightly  when  their  solutions  are  placed  between  the  plati- 
num wires ;  phosphoric  and  acetic  acids,  which  are  known  to 
be  weaker  acids,  cause  the  lamp  to  glow  with  a  dull  red,  show- 
ing that  these  acids  conduct,  but  to  a  much  smaller  extent 
than  the  strong  acids ;  carbonic  acid,  which  we  know  as  a 
very  weak  acid,  does  not  conduct  sufficiently  to  cause  the 
lamp  to  glow,  but  still  it  does  conduct  to  some  extent,  as  could 
be  shown  with  a  more  delicate  instrument  for  measuring  the 
passage  of  the  electric  current.  Solutions  of  the  strong 
bases,  sodium  hydroxide  and  potassium  hydroxide,  give  a 
brilliant  glow  to  the  lamp,  showing  that  they  conduct  the 
current  to  about  the  same  degree  as  solutions  of  salts  and 
of  strong  acids.  A  solution  of  the  weak  base  ammonium 
hydroxide  gives  a  dull  red  glow  to  the  filament,  and  thus 
conducts  to  about  the  same  extent  as  the  weaker  acids. 

319.  Non-Electrolytes.     Solutions  of  some  substances,  for 
example  sugar  and  alcohol,  do  not  conduct  any  better  than 
pure  water.      Such  substances  are  called  non-electrolytes,  in 
distinction  to  electrolytes  which  do  conduct  when  dissolved. 

320.  Character  of  Electrolytic  Conduction.     All  dry,  solid 
salts  are  like  dry  sodium  chloride  in  that  they  do  not  conduct 
electricity.     Pure  acids  and  pure  bases  when  entirely  free 
from  water  are  also  non-conductors.     All  become  conduc- 
tors when  they  are  dissolved  in  water,  but  the  manner  in 
which  the  conduction  takes  place  differs  strikingly  from  the 
manner  in  which  it  takes  place  in  metals.     In  metals,  abso- 
lutely no  permanent  change  takes  place  in  the  conductor, 
but  in  solutions  the  passage  of  the  current  is  accompanied 

B.  AND  W.  CHEM. 20 


304  THE   IONIC  THEORY 

by  a  decomposition  of  either  the  dissolved  substance  or  the 
water.  The  passage  of  electricity  through  a  solution  to- 
gether with  the  accompanying  decomposition  is  known  as 
electrolysis,  a  term  which  we  have  already  used  a  few  times. 

321.  The  Ionic  Theory.    It  was  in  the  effort  to  explain  the 
remarkable  facts  concerning  electrolytic  conduction  that  the 
theory  of  ions  was  developed.     The  theory  in  its  present 
form  was  proposed  by  Arrhenius,  a  Swedish  physicist,  in 
1887,  and  was  called  by  him  the  electrolytic  dissociation  theory; 
it  is  now  more  commonly  known  as  the  ionic  theory. 

According  to  the  ionic  theory,  when  a  salt  —  or  acid,  or 
base  —  is  dissolved  in  water,  its  molecules  become  disso- 
ciated, or  separated,  into  two  components  called  ions.  These 
ions  are  heavily  charged  with  positive  and  negative  electricity, 
respectively.  The  process  of  ionization  may  be  represented 
by  an  equation  in  the  same  manner  as  an  ordinary  chemical 
reaction,  for  example  the  ionization  of  sodium  chloride  takes 

place  as  follows : 

NaCl  -+  Na+  +  Cl". 

The  electric  charges  are  represented  by  the  symbols"*" 
and". 

322.  Pairing  of  Ions.     The  ions  are  separated  by  the  action 
of  the  water  so  that  they  are  free  to  move  about  in  the  solution 
to  some  extent  independent  of  each  other ;  but,  of  course,  the 
attraction  between  the  unlike  charges  of  electricity  is  so  great 
that  a  positive  ion  cannot  be  drawn  away  from  the  immediate 
neighborhood  of  its  accompanying  negative  ion  unless  there  is 
another  negative  ion  near  which  will  attract  it,  while  at  the 
same  time  there  is  another  positive  ion  near  to  attract  the 
now  unbalanced  negative  ion.     This  amounts  to  saying  that 
every  positive  ion  must  at  all  times  be  paired  off  with  a  nega- 
tive ion,  but  that  the  attachment  of  the  ions  in  the  pair  is  not 


DEGREE  OF  IONIZATION  305 

/    so  firm  as  to  prevent  a  continual  interchange  of  partners,  so 

)     to  speak. 

L-  323.  Reactivity  of  Ionized  Substances.  In  non-ionized 
substances,  we  may  suppose,  if  we  like,  that  the  two  com- 
ponents still  possess  the  electric  charges,  but  that,  without 
the  action  of  water  to  separate  the  components,  they  remain 
so  firmly  bound  together  in  ihe  molecule  that  this  interchange 
of  partners  is  not  possible.^ 

It  is  a  well-known  fact  that  dry,  solid  substances  are  inac- 
tive when  cold.  For  example,  baking  powder,  which  is  a 
mixture  of  a  dry  powdered  acid  substance  with  dry  sodium 
bicarbonate,  remains  without  action  indefinitely  so  long  as  it 
is  kept  dry.  WThen  it  is  moistened,  on  the  other  hand,  it 
reacts  immediately.  The  acid  substance  dissolving  in  the 
water  gives  H+  ions  and  the  sodium  bicarbonate  gives  HCOs" 
ions.  These  ions  enter  immediately  into  reaction,  leaving 
their  former  partners  and  pairing  with  each  other,  whereby 
they  form  carbonic  acid  and  so  produce  the  effervescence : 

H+  +  HC03-  ->  H2CO3  -t  H20  +  C02  f. 

324.  Electrolytic  Conduction.     When  an  electric  current 
passes  through  a  solution,  it  does  not  pass  in  a  continuous 
stream,  as  water  through  a  pipe,  but  it  travels  only  upon  the 
ions,  which  act  as  carriers.     Each  ion  bears  a  definite  load 
of  electricity,  the  positive  ion  a  load  of  .positive  electricity 
which  it  carries  through  the  solution  to  the  negative  electrode, 
and  the  negative  ion  a  load  of  negative  electricity  which  it 
carries  to  the  positive  electrode. 

325.  Degree  of  lonization.     Some  substances  in  solution 
conduct  electricity  strongly.     It  seems  fair  to  assume  that 
these  substances  have  yielded  a  large  number  of  carriers,  that 
is,  of  ions.     As  a  matter  of  fact,  there  are  good  reasons  for 


306  THE   IONIC   THEORY 

believing  that  salts  and  strong  acids  and  strong  bases  do  yield 
a  large  number  of  ions  when  dissolved ;  the  great  majority 
of  the  molecules  are  dissociated.  On  the  other  hand,  only  a 
small  proportion  of  the  molecules  in  the  weak  acids  and  bases 
are  split  up  into  ions ;  the  rest  of  the  molecules  remain  un- 
dissociated  and  incapable  of  taking  part  in  the  conduction  of 
electricity.  Thus  in  a  solution  of  acetic  acid,  which  we  have 
said  is  a  weak  acid,  it  is  estimated  that  about  one  molecule 
in  every  one  hundred  is  dissociated,  whereas  the  other  ninety- 
nine  molecules  are  intact.  We  attribute  acid  properties 
to  the  presence  of  hydrogen  ions,  and  it  may  be  seen  that 
according  to  the  theory  the  weakness  of  the  acid  character 
of  acetic  acid  and  the  feebleness  of  its  conductance  arise  from 
the  same  cause,  namely,  from  a  disinclination  or  inability 
of  its  molecules  to  break  apart  into  ions. 

326.  Value  of  Ionic  Theory.     When  Arrhenius  originated 
the  ionic  theory,  he  was  led  to  his  conclusions,  not  only  by  the 
phenomena  of  conduction  and  by  the  similarity  of  properties 
of  all  acids  and  of  all  bases,  but  also  by  a  number  of  other 
striking  properties  of  conducting  solutions.     It  is  beyond  the 
scope  of  this  book  to  give  all  of  the  reasons  for  adopting  the 
ionic  theory ;  but  it  may  be  said  that  it  is  believed  to-day  by 
most  chemists  that  the  ionic  theory  gives  a  reasonably  correct 
picture  of  the  conditions  in  solutions  that  conduct  electricity. 
Without  this  theory  it  would  be  difficult  to  account  for 
electrolytic  conduction  or  for  the  fact  that  all  acids  have  the 
same  acid  properties  and  all  bases  have  the  same  basic  prop- 
erties.    We  make  use  of  the  ionic  theory  because  it  is  a  great 
aid  in  helping  us  to  understand  chemical  principles  and  to 
systematize  our  knowledge  of  the  chemistry  of  solutions. 

327.  Neutralization  According  to  Ionic  Theory.     One  of 
the  greatest  services  of  the  ionic  theory  is  that  it  has  given  us 


NEUTRALIZATION  ACCORDING  TO  IONIC  THEORY     307 

a  far  clearer  understanding  of  neutralization  reactions.  We 
have  said  that  the  neutralization  of  an  acid  and  a  base  results 
in  the  formation  of  water  and  a  salt  with  a  disappearance  of 
the  characteristic  properties  of  acid  and  base.  According 
to  the  ionic  theory  an  acid  when  in  solution  consists  of  a  hy- 
drogen ion  and  a  radical  ion  and  a  base  consists  of  a  metal 
ion  and  an  hydroxyl  ion.  Thus  hydrochloric  acid  is 

H+  +  Gl- 
and sodium  hydroxide  is 

Na+  +  OH-. 

Pure  water  does  not  conduct  electricity  and  therefore  does  not 
contain  ions  but  only  molecules  of  H^O  (or  HOH).  "  Now  on 
bringing  acid  and  base  together,  H+  ions  and  OH~  ions  come 
into  contact.  They  are  components  of  water,  but  water 
cannot  exist  in  ionized  form ;  in  other  words,  these  compo- 
nents cannot  continue  to  exist  separately  when  in  presence  of 
each  other,  they  combine  to  form  undissociated  water  mole- 
cules, H+  +  QH-  ^  (HQH)  or  H2Q 

Sodium  chloride,  on  the  other  hand,  we  know  to  exist  in  solu- 
tion in  the  form  of  ions.  Hence  the  Cl~  ion  of  the  acid  and 
the  Na+  ion  of  the  base  do  not  suffer  any  change  during  the 
neutralization  but  simply  remain  in  the  neutralized  solution. 
The  complete  neutralization  reaction  would  thus  appear  as 
follows : 

H+  +  Cr  +  Na+  +  OH-  -+  HOH  +  Na+  +  CT. 

The  resulting  solution  is  identical  with  one  that  would  be  ob- 
tained by  dissolving  solid  sodium  chloride  in  water,  hence  we 
are  perfectly  correct  in  speaking  of  it  as  a  sodium  chloride 
solution.  If  this  solution  is  evaporated,  water  escapes  as 
water  vapor  and  the  Na+  ions  and  Cl~  ions,  no  longer  being 


308  THE  IONIC  THEORY 

held  apart  by  the  water,  join  to  form  molecules  of  sodium 
chloride.  When  the  water  is  all  evaporated,  nothing  is  left 
but  solid  sodium  chloride. 

It  is  thus  true  that  water  and  a  salt  are  the  products  of 
neutralization,  but  it  must  be  borne  in  mind  that  undisso- 
ciated  water  is  always  formed,  whereas  undissociated  salt  is 
usually  not  formed  unless  the  water  is  first  driven  off. 

SUMMARY 

Metallic  Conductance :  All  metals  conduct  electricity  without 
suffering  perceptible  changes. 

Non- Conductors :  Dry  non-metallic  substances,  including  com- 
pounds of  metals,  do  not  conduct  electricity. 

Electrolytic  Conductance :  Pure  water  is  a  non-conductor  of  elec- 
tricity. Dry  acids,  bases,  and  salts  also  are  non-conductors. 
But  water  solutions  of  acids,  bases,  and  salts  conduct.  Elec- 
trolytic conduction,  however,  is  different  from  metallic  conduc- 
tion in  that  solutions  of  acids,  bases,  and  salts  undergo  chemical 
change  during  the  passage  of  an  electric  current. 

The  ionic  theory  assumes  that  when  an  acid,  base,  or  salt  is  dissolved 
in  water,  its  molecules  become  separated  into  two  components 
called  ions.  Furthermore,  the  ions  are  heavily  charged  with 
positive  and  negative  electricity,  respectively. 

Attraction  and  Repulsion :   Bodies  bearing  like  electric  charges  repel 

each  other,  whereas  bodies  with  unlike  charges  attract. 
Thus  positively  charged  ions  in  solution  attract  and  hold  an  elec- 
trically equivalent  number  of  negatively  charged  ions.  The 
ions  are  free  to  move  past  each  other  and  they  are  all  the  time 
exchanging  partners,  but  it  is  not  possible  for  ions  of  one  kind 
to  exist  in  solution  except  when  balanced  by  ions  of  opposite 
charge  in  their  immediate  vicinity. 

Ions  convey  the  electric  current  which  passes  through  a  solution. 

Hydrogen  ions  are  responsible  for  the  properties  of  acids. 

Hydroxyl  ions  are  responsible  for  the  properties  of  bases. 

Salts,  strong  acids,  and  strong  bases  conduct  strongly  and  are  highly 
ionized  when  dissolved  in  water. 


QUESTIONS  309 

Weak  acids  and  weak  bases  contain  proportionately  fewer  ions  than 
strong  acids  and  bases.  Thus  the  conductance  for  electricity 
and  the  acid  and  basic  properties  are  weaker. 

Substances  enter  readily  into  reaction  when  in  the  ionized  condi- 
tion, because  their  components  are  separated  and  thus  free  to 
exchange  partners.  When  un-ionized,  the  components  are 
rigidly  bound  together  and  substances  do  not  readily  enter  into 
reaction. 

Neutralization  according  to  the  ionic  theory  consists  in  the  pairing 
of  positive  hydrogen  ions  of  the  acid  with  negative  hydroxyl 
ions  of  the  base  to  form  undissociated  water.  The  negative 
radical  ions  of  the  acid  and  the  positive  metal  ions  of  the  base 
remain  in  the  solution  as  a  dissociated  salt.  The  undissociated 
dry  salt  is  obtained  by  evaporating  off  the  water. 

Questions 

1.  In  what  important  respect  is  acid  hydrogen  different  from 
ordinary  hydrogen?     To  what  is  this  difference  attributed? 

2.  What  component  of  all  bases  causes  litmus  to  become  blue? 
of  all  acids  causes  litmus  to  become  red  ? 

3.  Why  do  solutions  of  acids,  bases,  and  salts  conduct  the  electric 
current,  whereas  the  dry  substances  are  non-conductors? 

4.  Explain  in  terms  of  the  ionic  theory  why  carbonic  acid  is  so 
much  weaker  than  sulphuric  acid. 

5.  What  argument  can  you  advance  in  favor  of  the  truth  of  the 
ionic  theory,  other  than  those  drawn  from  the  conductivity  of  solu- 
tions of  electrolytes? 

6.  Formulate  in  terms  of  the  ionic  theory  what  takes  place  during 
the  neutralization  of 

KOH  and  HC1  Ca(OH)2'and  H2S04 

NaOH  and  H2S04  NH4OH  and  H2S04 

KOH  and  H2SO4  NH4OH  and  HC1 

Ca(OH)2  and  HC1  KOH  and  HN03 

7.  Remembering  that  silver  chloride  is  insoluble,  write  ionized 
equations  for  the  following  cases 

AgN03  +  HC1  -^  ? 
Ag2S04  +  KCi  ->  ? 


310  THE   IONIC   THEORY 

8.  Remembering  that  barium  sulphate  is  insoluble,  write  ionized 
equations  for  the  following  cases 

BaCl2  +  Na2S04  ->•  ? 
Ba(OH)2  +H2S04-»? 

9.  Remembering  that  all  sodium,  potassium,  and  ammonium  salts 
are  soluble,  write  ionized  equations  for  the  following  cases 

NaOH  +  HNO3  -+  ? 

NH4C1  +  NaN03  ->  ? 

KN03  +  NaCl  -»  ? 

NH4C1  +  NaOH  ->  ? 

NH4OH  +  HC1  ->  ? 


CHAPTER  XXVI 


ELECTROLYSIS 

328.  Electrolysis  of  Hydrochloric  Acid.  When  a  solution 
of  hydrochloric  acid  is  electrolyzed,  hydrogen  gas  is  given  off 
at  the  negative  pole  and  chlorine  gas  at  the  positive  pole 
(Chapter  XIII).  The  poles,  or  electrodes,  should  consist  of 
strips  of  platinum, 
because  platinum  is 
attacked  but  little  by 
either  the  acid  or  the 
chlorine.  The  elec- 
trodes are  immersed 
at  opposite  sides  of 
the  solution  and  they 
are  connected  by 
metal  wires  with  the 
electric  cell  or  the 
dynamo  (Fig.  61). 
Thus  one  electrode  is  kept  constantly  charged  with  posi- 
tive electricity  and  the  other  with  negative  electricity. 
Since  chlorine  is  given  off  at  the  positive  pole,  the  chlorine 
must  first  have  been  attracted  through  the  solution  to 
the  surface  of  the  pole,  and  since  it  is  attracted  to  a  positively 
charged  body,  it  must  itself  be  negatively  charged.  By  the 
same  reasoning,  since  hydrogen  is  given  off  at  the  negative 
electrode,  the  hydrogen  in  the  solution  must  be  positively 
charged. 

311 


FIG.  61.  —  Electrolysis  of  Hydrochloric  Acid. 


312  ELECTROLYSIS 

v 

329.  Discharge  of  Ions.  It  is,  of  course,  obvious  that 
ordinary  hydrogen  gas  and  chlorine  gas  are  not  electrically 
charged.  Before  their  escape  from  the  surface  of  the  elec- 
trodes, the  hydrogen  and  chlorine  ions,  respectively,  must 
have  surrendered  their  electric  charges, 

+          © 


Cl" 

The  electric  charges  thus  given  up  to  the  electrodes  neutralize 
the  opposite  charges  which  are  being  continually  supplied 
from  the  dynamo. 

330.  Ions  as  Carriers.  The  ions  which  come  up  to  the 
electrodes  during  the  electrolysis  and  give  up  their  charges 
are  bringing  a  steady  supply,  or  current,  of  electricity.  They 
may  be  likened  to  a  line  of  laborers  each  with  a  wheelbarrow 
load  of  gravel.  Each  laborer  comes  up  in  turn,  discharges 
his  load  of  gravel  in  the  desired  place,  and  steps  aside  with  his 
wheelbarrow.  In  the  same  way  a  file  of  H+  ions  moves 
towards  the  negative  electrode.  As  each  ion  reaches  the 
electrode,  it  discharges  its  load  of  electricity  and  retires  to 
form  hydrogen  gas  (by  doubling  up  with  the  next  hydrogen 
atom  released).  A  file  of  Cl~  ions  moves  at  the  same  time  in 
the  opposite  direction,  and  each  ion  in  turn  gives  up  its  charge 
at  the  positive  electrode  and  retires  to  form  ordinary  chlorine 
gas  (by  pairing  with  the  next  atom  of  chlorine). 

Let  us  picture  the  process  by  means  of  diagrams.  Let 
a  white  circle  O  represent  a  hydrogen  atom  and  a  circle  with 

a  +  sign  O  represent  a  hydrogen  ion.  Let  a  black  circle  • 
represent  a  chlorine  atom  and  •  a  chlorine  ion.  At  the  out- 
set let  us  represent  six  of  each  kind  of  ions  in  the  solution  be- 


IONS   AS   CARRIERS 


313 


tween  the  electrodes  instead  of  the  countless  number  which 
is  actually  present  (Fig.  62).  After  the  current  has  passed 
for  some  time,  a  certain  number  of  ions  —  let  us  say  two  of 
each  kind  —  has  been  discharged.  The  two  H+  ions  nearest 
the  negative  electrode  have  moved  up  to  it  and  discharged. 


0 


o 


o 


0 


o 


o 


FIG.  62.  —  Ions  in  Solution  before  Current  has  passed  between  Electrodes. 

This  leaves  unbalanced  the  two  Cl~  ions  which  were  originally 
paired  with  the  two  H+  ions.  It  is  impossible  for  them  to 
remain  long  unbalanced.  They  are  repelled  by  the  nega- 
tively charged  electrode  and  they  move  away  from  it,  but  in 
moving  away  they  repel  the  four  Cl~  ions  in  front  of  them. 
At  the  same  time  they  attract  the  hydrogen  ions  farther  down 
the  line  and  these  move  up  to  balance  them.  At  the  other 
electrode  a  similar  process  has  taken  place ;  two  chlorine 
ions  have  discharged  and  left  two  hydrogen  ions  unbalanced. 
The  latter  are  repelled  and  move  away,  following  the  other 
hydrogen  ions.  At  this  point  in  the  electrolysis  we  have 


I 


+ 

O 


o 


+ 

o 


+ 

o 


8 


FIG.  63.  —  Showing  Condition  of  Solution  from  Fig.  62,  after  Current  has  passed  for 

a  Time. 

positive  and  negative  ions  balanced  against  each  other 
throughout  the  solution  just  as  at  the  outset,  but  there  are 
only  four  pairs  now  instead  of  six  as  at  first  (Fig.  63) ; 


314  ELECTROLYSIS 

and  accumulated  on  the  poles  we  have  a  molecule  of  hydrogen 
and  a  molecule  of  chlorine,  respectively. 

If  the  electrolysis  were  continued  long  enough,  all  the  ions 
would  be  withdrawn  from  the  solution  and  nothing  but  pure 
water  would  be  left.  In  such  a  case,  of  course,  the  current 
would  stop  flowing,  for  there  would  be  nothing  left  to  conduct 
it ;  for  pure  water  is  a  non-conductor. 

331.  Atomic  Structure  of  Electricity;  Electrons.  It  has 
probably  occurred  to  the  reader  to  wonder  what  determines 
just  how  much  electricity  each  ion  carries.  Evidently  the 
positive  charge  of  the  hydrogen  ion  is  just  equal  and  opposite 
to  the  negative  charge  of  the  chlorine  ion ;  and  it  is  almost 
equally  evident  that  every  hydrogen  ion  always  has  just  the 
same  amount  of  positive  electricity  and  every  chlorine  ion 
has  just  the  same  amount  of  negative  electricity.  One  might 
well  wonder  if  electricity  does  not  have  an  atomic  structure 
just  like  ordinary  matter  and  if  the  charge  on  the  hydrogen 
ion  is  not  really  a  single  atom  of  positive  electricity  and  the 
charge  on  the  chlorine  ion  a  single  atom  of  negative  electricity. 
That  this  is  really  so  seems  extremely  probable  in  the  light 
of  our  present  knowledge,  and  we  shall  from  now  on  speak  of 
the  electric  atoms  as  confidently  as  we  speak  of  the  atoms  of 
the  material  elements.  We  shall  call  these  smallest  divisions 
of  electricity  electrons  and  we  shall  distinguish  positive  and 
negative  electrons  and  give  them  the  symbols  0  and  0 . 

Recent  study  of  electric  discharges  in  vacuum  tubes,  and 
work  with  the  wonderful  element  radium  have  thrown  a 
great  deal  of  light  on  the  subject  of  electrons.  The  negative 
electrons  have  actually  been  discovered,  unassociated  with 
ordinary  matter,  in  the  discharge  from  the  negative  electrode 
in  vacuum  tubes.  Positive  electrons  have  never  been  found 
except  in  association  with  atoms  of  the  ordinary  elements. 


ELECTROLYSIS  OF  SULPHURIC  ACID  315 

It  may  be  that  only  what  we  call  the  negative  electrons  have 
a  real  existence  and  that  what  we  call  positive  electrons  are 
merely  atoms  of  ordinary  matter  deprived  of  negative  elec- 
trons which  belong  there.  We  shall  find  it  convenient  to  speak 
of  both  positive  and  negative  electrons  as  well  as  of  positive 
and  negative  electricity  as  if  both  existed,  and  we  shall  not 
concern  ourselves  with  the  question  as  to  whether  there  are 
really  two  kinds  or  only  one  kind  of  electricity. 

332.  Electrolysis  of  Sulphuric  Acid.  Sulphuric  acid  con- 
tains hydrogen  and  a  sulphate  radical.  Since  the  acid  gives 
H+  ions  when  it  dissociates,  the  sulphate  radical  must  form 
the  negative  ions  : 


When  this  acid  is  electrolyzed,  it  might  be  expected  that 
SO4  ions  would  travel  towards  the  positive  electrode  and 
H+  ions  towards  the  negative  electrode.  As  a  matter  of  fact, 
hydrogen  is  given  off  at  the  negative  electrode,  but  at  the  other 
electrode  oxygen  is  set  free  instead  of  the  SO4  radical. 

The  electrolysis  of  most  substances  is  a  more  complicated 
phenomenon  than  that  of  hydrochloric  acid,  in  which  the  ions 
that  carry  the  current  simply  discharge  and  the  respective 
elements  escape  in  the  uncombined  condition.  The  SO4 
ion  of  sulphuric  acid  is  actually  the  carrier  of  the  negative 
electricity,  but  the  uncharged  SO4  radical  is  incapable  of 
existence  alone.  W7hen  it  parts  with  its  'electricity  it  would 
either  itself  have  to  break  down  into  other  substances  or  it 
would  react  with  the  solvent  water.  The  latter  is  probably 
what  happens  and  it  is  thus  the  oxygen  of  the  water  that 
escapes. 

S04  +  H20  ->  H2S04  +  O 

2  0  —  >  O2  (molecule  of  oxygen  gas  which  escapes)  . 


316 


ELECTROLYSIS 


The  sulphuric  acid  thus  formed  in  this  secondary  reaction  of 
course  dissociates  immediately  into  ions  under  the  influence 
of  the  water.  jj^  _^  2  H+  +  SOT  ~. 

333.  It  will  be  remembered  that  in  decomposing  water 
by  the  electric  current  (page  103),  sulphuric  acid  was  added 
in  order  to  make  the  solution  conduct.  It  was  then  stated 
that  the  sulphuric  acid  suffered  no  change  but  that  only  the 
water  was  decomposed,  and  it  is  now  seen  why  the  statement 
was  justified.  It  is  really  the  ions  of  sulphuric  acid  that  carry 

the  current,  but  for  every  molecule 
of  sulphuric  acid  that  is  drawn 
apart  another  molecule  is  generated 
by  the  secondary  reaction  at  the 
positive  electrode,  and  the  amount 
of  sulphuric  acid  in  the  whole  solu- 
tion suffers  no  change. 

334.  If  what  has  just  been  said 
is  true,  there  must,  however,  be 
an  accumulation  of  sulphuric  acid 
around  the  positive  electrode  and 
a  depletion  around  the  negative 
electrode,  for  the  SO4  ions  are 
drawn  away  from  the  latter  and  the 
H+  ions  are  discharged  there.  We 
ought  to  be  able  to  show  this  ac- 
FIG.  64.  —  Electrolysis  of  Sui-  cumulation  and  depletion  in  order 

phuricAcid.    Arranged  to  prove     ,       .      ,.» 

the  passage  of  sulphate  ions   to  justify  our  arguments. 

In  the  apparatus  shown  in  Fig.  61 
the  solution  around  the  electrodes 
would  be  continually  in  circulation  and  on  this  account 
no  accumulation  of  sulphuric  acid  would  last  long  enough 


away  from   the   —    electrode 
and  towards  the  +  electrode. 


SECONDARY  REACTION  AT  ELECTRODES 


317 


to  be  demonstrated.  It  is  necessary  to  modify  the  apparatus 
somewhat  and  to  arrange  glass  cups  around  the  electrodes 
to  prevent  circulation  of  the  liquid  (Fig.  64).  Where  the 
sulphuric  acid  accumulates,  the  cup 
must  be  open  at  the  top  and  closed 
at  the  bottom  because  the  more  con- 
centrated a  solution  the  heavier  it  is. 
Where  the  sulphuric  acid  is  depleted, 
the  cup  must  have  an  opening  at  the 
bottom  for  the  passage  of  the  current 
and  the  top  must  rise  above  the  sur- 
face of  the  liquid  because  the  acid  of 
diminished  concentration  is  lighter 
than  the  original  acid. 

If  after  the  current  has  passed  for 
some  time  through  a  dilute  solution 
of  sulphuric  acid  in  this  apparatus, 
portions  of  the  solution  are  removed 
with  a  pipette  (Fig.  65)  from  the  two 
cups  and  analyzed,  it  is  actually  found 
that  the  one  from  the  cup  around  the  negative  electrode 
contains  less  sulphuric  acid  and  that  from  the  cup  around 
the  positive  electrode  more  sulphuric  acid  than  at  the  start. 

335.  Secondary  Reaction  at  Electrodes.  As  already 
stated,  it  is  not  always  true  that  the  products  which  appear 
at  the  electrodes  as  a  result  of  electrolysis  are  formed  directly 
from  the  ions  that  carry  the  current  through  the  solution. 
These  products  arise  quite  as  often  from  secondary  reactions 
which  the  discharged  ions  enter  into  either  among  themselves 
or  with  water.  It  is  hence  not  possible  to  say  from  an  ob- 
servation of  the  products  liberated  at  the  poles  what  are  the 
ions  in  the  solution.  In  the  electrolysis  of  sodium  hydroxide 


FIG.  65.  —  Pipette. 


318  ELECTROLYSIS 

and  sodium  sulphate,  for  example,  the  products  liberated  are 
hydrogen  and  oxygen.  These  gases  are  evolved  in  the  pro- 
portion of  two  volumes  of  the  former  to  one  of  the  latter, 
just  as  in  the  electrolysis  of  dilute  sulphuric  acid.  We  have 
thus  a  typical  acid,  a  typical  base,  and  a  typical  salt  all  of 
which  yield  as  the  final  products  of  their  electrolysis  only 
the  two  constituents  of  pure  water.  Yet  these  substances 
cannot  all  have  the  same  ions. 

336.  Electrolysis  of  Sodium  Hydroxide.  It  has  already 
been  stated  that  the  ions  of  sodium  hydroxide  are  Na+  and 
OH~  and  it  must  be  these  that  carry  the  current.  At  the 
negative  electrode  we  may  imagine  the  Na+  ion  to  be  dis- 
charged,  Na+  -*  Na  +  0 

and  the  sodium  atom  to  react  with  water, 

2  Na  +  2  HOH  -^  2  NaOH  +  H2. 

At  the  positive  electrode,  we  may  imagine  that  the  OH~  ions 
discharge, 


and  that  the  hydroxyl  groups  so  formed  react  together  to 
give  water  and  oxygen  gas  : 

4  OH  ->  2  H2O  +  O2. 

We  have  taken  for  granted  that  hydrogen  is  only  the  sec- 
ondary product  of  the  electrolysis  and  that  it  results  from  the 
reaction  of  the  water  on  the  sodium  which  is  first  set  free. 
If  it  were  possible  to  electrolyze  sodium  hydroxide  without 
using  water  as  a  solvent,  it  might  be  possible  to  obtain  metal- 
lic sodium  instead  of  hydrogen  at  the  negative  electrode. 
Dry,  solid  sodium  hydroxide  does  not  conduct  electricity; 
but  if  heated  strongly  it  melts,  and  the  liquid  sodium  hydrox- 
ide is  an  excellent  electrolytic  conductor.  Metallic  sodium 


ELECTROLYSIS  OF  SODIUM  SULPHATE       319 

is  then  obtained  as  a  deposit  at  the  negative  electrode,  and 
this  is  one  of  the  methods  by  which  metallic  sodium  is  manu- 
factured on  a  commercial  scale. 

337.  Electrolysis  of  Sodium  Sulphate.  Sodium  sulphate 
is  the  salt  obtained  by  neutralizing  sodium  hydroxide  with 
sulphuric  acid,  and  its  solution  contains  the  same  metal  ion 
as  the  base  and  the  same  radical  ion  as  the  acid.  Thus  on 
electrolyzing  a  sodium  sulphate  solution,  since  the  negative 
ion  is  the  same,  the  same  change  occurs  at  the  positive  pole 
as  when  sulphuric  acid  is  electrolyzed,  namely,  the  ion  dis- 
charges and  the  sulphate  radical  reacts  with  water, 

2  S04  +  2  H20  ->  2  H2SO4  +  O2. 

At  the  negative  electrode  the  same  change  occurs  as  when 
sodium  hydroxide  is  electrolyzed,  namely,  the  sodium  ion 
discharges  and  the  metal  atom  then  reacts  with  water, 
2  Na  +  2  HOH->2  NaOH  +  H2. 

Sodium  sulphate  is  a  neutral  salt  and  its  solution  does  not 
change  the~color  of  either  red  or  blue  litmus.  If  the  solution 
is  electrolyzed,  however,  in  an  apparatus  similar  to  that 
shown  in  Fig.  64,  in  which  both  electrodes  are  surrounded  by 
cups,  a  piece  of  litmus  placed  in  the  cup  around  the  positive 
pole  will  immediately  turn  red,  showing  that  an  acid  is  formed, 
and  the  piece  of  litmus  in  the  other  cup  will  immediately  turn 
blue,  showing  that  a  base  is  produced.  By  glancing  back  to 
the  equations  for  the  secondary  reactions,  one  sets  that  in 
accounting  for  the  formation  of  the  hydrogen  and  oxygen,  it 
was  necessary  to  allow  also  for  the  formation  of  sodium  hy- 
droxide and  sulphuric  acid  at  the  respective  electrodes.  The 
formation  of  base  and  acid  at  the  two  poles  agrees,  therefore, 
with  our  interpretation  of  the  secondary  reactions  and  in- 
creases our  confidence  in  the  correctness  of  our  deductions. 

B.  AND  W.  CHEM. 21 


320  ELECTROLYSIS 

If  the  reactions  are  studied  a  little  further,  it  will  be  seen 
that  since  two  atoms  of  hydrogen  are  liberated  for  each  atom 
of  oxygen,  two  molecules  of  sodium  hydroxide  should  be 
formed  at  the  one  pole  for  each  molecule  of  sulphuric  acid 
at  the  other.  These  are  equivalent  quantities,  and  should 
just  neutralize  each  other.  Now  if  to  test  this  deduction  the 
whole  solution  is  thoroughly  mixed  after  the  electrolysis  has 
been  stopped,  it  is  found  that  the  solution  becomes  again  per- 
fectly neutral  and  will  not  change  either  red  or  blue  litmus. 
Thus  there  are  in  fact  exactly  equivalent  quantities  of  acid 
and  base  produced  by  the  electrolysis  of  the  salt. 

338.  Electrolysis  of  Sodium  Chloride.     One  of  the  im- 
portant commercial  methods  of  manufacturing  sodium  hy- 
droxide is  by  the  electrolysis  of  sodium  chloride,  the  most 
abundant  and  least  expensive  compound  of  sodium.     The 
sodium  hydroxide  is  obtained  at  the  negative  electrode,  and 
chlorine,  which  is  an  even  more  valuable  product,  is  obtained 
at  the  other  electrode  (see  Chlorine,  Chapter  XVI). 

Sodium  is  one  of  the  most  active  of  all  metals  and  it  is  be- 
cause of  this  that  sodium  does  not  appear  as  one  of  the  prod- 
ucts when  solutions  of  sodium  salts  are  electrolyzed.  No 
more  do  we  obtain  potassium  or  calcium  when  solutions  of 
their  salts  are  electrolyzed ;  we  obtain,  instead,  the  bases 
potassium  hydroxide  and  calcium  hydroxide,  just  as  we  ob- 
tained sodium  hydroxide  by  the  electrolysis  of  a  sodium 
salt. 

339.  Electrolysis  of  Copper  Sulphate.     When,  however,  a 
salt  of  a  metal  which  does  not  decompose  water  is  electrolyzed, 
the  metal  itself  ought  to  be  obtained.      For  example,  if  the 
current  is  passed  through  a  solution  of  copper  sulphate, 
CuSO4,  between  platinum  electrodes,  a  reddish  metallic  de- 
posit of  copper  begins  at  once  to  appear  on  the  negative  pole, 


ELECTROPLATING  OF  COPPER  321 

while  at  the  other  pole  oxygen  escapes.  The  copper  ions 
simply  surrender  their  charges  to  the  electrode, 

Cu++  ->  Cu  +  2  © 

and  the  unelectrified  atoms  stick  there  and  soon  build  up  a 
coherent  layer  of  the  metal.  At  the  other  electrode,  the  ions 
which  come  up  to  discharge  are  the  same  as  with  sulphuric 
acid  or  sodium  sulphate,  and  so  long  as  the  electrodes  are  of 
the  unattackable  metal  platinum,  the  reaction  is  the  same  as 
already  described  under  the  electrolysis  of  these  substances. 

340.  The  electroplating  of  copper  is  an  art  of  very  great 
importance.     The  object  to  be  covered  is  made  the  negative 
electrode  in  a  bath  of  a  copper  salt,  frequently  copper  sul- 
phate ;   for  the  other  electrode  a  thick  plate  of  copper  is  used 
instead  of  platinum  or  other  unattackable  metal.     Now, 
when  the  current  passes,  Cu++  ions  discharge  at  one  electrode 
and  this  would,  after  a  while,  deplete  the  bath  of  copper  ex- 
cept that  the  metal  of  the  other  electrode  dissolves  and  main- 
tains the  supply.     The  SC>4       ions  are  attracted  to  the  posi- 
tive electrode  as  usual,  but  instead  of  having  to  give  up  their 
charges,  they  become  electrically  balanced  by  new  copper 
ions  which  are  formed  from  the  metal  of  the  electrode  and  the 
positive  electricity. 

Cu  +  2  ©  ->  Cu++. 

Thus  electroplating  consists  merely  in  transferring  copper 
in  the  form  of  ions  through  a  solution  from  a  thick  mass  of 
the  metal  to  the  object  to  be  plated.  (Compare  Electrolytic 
Refining  of  Copper,  page  254.) 

341.  Multiple  Proportions  among  Ionic  Charges.     Of  the 
several  ions  mentioned  in  this  and  the  preceding  chapter,  it 
is  noticed  that  some  are  represented  as  carrying  single  charges 
of  electricity,  or  single  electrons,  whereas  others  are  repre- 


322  ELECTROLYSIS 

sented  as  having  exactly  twice  as  much  electricity,  that  is, 
two  electrons  each.  Following  is  a  table  giving  some  of  the 
very  common  ions  and  showing  the  number  and  character  of 
the  charges  on  each  :  , 

Hydrogen  .  \     .     .     .  H+  Chloride    .     .     .     .     Cl~ 

Sodium       .     .         ,  Na+          Nitrate      ....     N03~ 

Potassium       .     .     .     .  K+  Bromide     ....     Br~ 

Ammonium     ....  NH4+ 

Silver Ag+ 

Calcium Ca++         Sulphate    .     .     .     .     S04~ 

Copper Cu++         Carbonate      .     .     .     C0s~ " 

Barium Ba++ 

Zinc       ......  Zn++ 

Magnesium    ....  Mg++ 

Aluminium     ....  A1+++        Phosphate      .     .     .     P04~- 

The  fact  that  the  amount  of  electricity  on  an  ion  is  always 
some  whole  multiple  of  the  unit  amount  that  resides  on  the 
H+  ion  or  the  Cl~  ion  was  originally  the  reason  for  wanting 
to  ascribe  an  atomic  structure  to  electricity.  If  electricity 
were  continuous  in  nature  and  not  atomic,  it  would  be  hard 
to  see  why  some  ions  should  bear  exactly  the  unit  charge, 
some  exactly  twice,  and  some  exactly  three  times  the  unit 
charge,  but  none  should  ever  bear  fractional  amounts.  As 
already  stated,  page  314,  the  idea  of  the  atomic,  or  electronic, 
structure  of  electricity  is  now  firmly  established. 

342.  Faraday's  Law.  It  was  observed  as  long  ago  as  early 
in  the  nineteenth  century  that  when  chemical  decompositions 
were  caused  by  the  passage  of  the  current,  there  was  a 
simple  relation  between  the  amounts  of  different  substances 
decomposed  by  the  same  quantity  of  electricity.  The  rela- 
tion was  first  clearly  recognized  by  Faraday,  and  the  law 
which  expresses  this  relation  is  known  as  Faraday's  law. 


FARADAY'S  LAW 


323 


We  may  state  the  law  as  follows :  The  passage  of  the  same 
amount  of  electricity  causes  the  liberation  of  equivalent  amounts 
of  different  substances  at  the  electrodes.  We  have  already  seen 
that  two  volumes  of  hydrogen  are  liberated  at  one  electrode 
while  one  volume  of  oxygen  is  set  free  at  the  other.  These 
are  equivalent  quantities,  for  they  are  the  amounts  which 
combine  with  each  other  to  form  water.  We  have  also  seen 
that  equivalent  amounts  of  acid  and  base  are  formed  at  the 
electrodes  when  a  salt  solution  is  electrolyzed. 

343.  When  an  electric  current  flows  in  a  given  circuit, 
it  is  true  that  the  quantity  which  passes  in  any  one  part  of 
the  circuit  is  exactly  the  same  as  that  which  passes  any  other 
part  of  the  circuit.  Let  us. construct  a  circuit  with  several 
electrolytic  cells  as  shown  in  Fig.  66,  the  first  cell  having 
silver  electrodes  in  silver  nitrate  solution,  the  second  copper 
electrodes  in  copper  sulphate  solution,  the  third  platinum 
electrodes  in  sulphuric  acid,  and  the  fourth  platinum  elec- 
trodes in  sodium  sulphate  solution. 


FIG.  66.  —  Electrolytic  Cells  in  Series. 

Let  us  now  allow  an  amount  of  electricity  to  pass  which 
is  sufficient  to  carry  108  grams  of  silver  from  the  positive 
electrode  into  the  solution  and  to  deposit  108  grams  of  silver 
on  the  negative  pole  of  the  first  cell,  and  let  us  observe  what 


324  ELECTROLYSIS 

corresponding  changes  take  place  in  other  parts  of  the  cir- 
cuit. 108  is  the  atomic  weight  number  of  silver  and  108 
grams  is  the  gram-atomic  weight. 

In  the  second  cell,  we  find  that  31.8  grams  of  copper  are 
dissolved  from  the  positive  electrode  and  that  an  equal 
amount  is  deposited  on  the  negative  electrode.  The  atomic 
weight  of  copper  is  63.6;  hence  31.8  grams  is  one  half  an 
atomic  weight  in  grams.  This  amount  of  copper,  then,  is 
equivalent  to  108  grams  or  a  whole  atomic  weight  in  grams 
of  silver,  electrically  as  well  as  chemically.  Chemically,  an 
atom  of  copper  is  equivalent  to  two  atoms  of  silver,  for  it  has 
twice  the  capacity  for  holding  atoms  of  non-metallic  elements 
in  combination.  This  is  shown,  for  example,  by  comparison 
of  the  oxides  and  chlorides : 

CuO  CuCl2 

Ag20  AgCl 

In  the  third  cell  of  the  series,  5.6  liters  of  oxygen  gas  (meas- 
ured under  standard  conditions)  are  liberated  at  the  positive 
electrode  and  11.2  liters  of  hydrogen  gas  at  the  negative 
electrode.  These  quantities  are  equivalent  to  the  amounts  of 
silver  and  copper  deposited  in  the  first  two  cells,  for  a  simple 
calculation  will  show  that  11.2  liters  of  hydrogen  weigh 
1  gram  and  thus  equal  one  gram-atomic  weight  of  that  ele- 
ment, and  that  5.6  liters  of  oxygen  weigh  8  grams  and  thus 
equal  one  half  gram-atomic  weight  of  the  latter  element. 

In  the  fourth  cell  of  the  series,  a  chemical  analysis  of  the 
solution  in  the  cup  around  the  negative  electrode  would 
show  that  40  grams  or  one  mole  of  sodium  hydroxide  is  pro- 
duced, and  an  analysis  of  the  solution  at  the  other  electrode 
would  show  that  49  grams  or  one  half  mole  of  sulphuric  acid 
is  produced.  These  amounts  are  equivalent  to  each  other 


SUMMARY  325 

and  to  the  amounts  of  silver,  copper,  hydrogen,  and  oxygen 
involved  in  the  other  cells. 

344.  The  foregoing  illustrations  have  shown  the  wide  appli- 
cation of  Faraday's  law.  Every  year  the  application  of 
electricity  in  chemical  manufactures  is  growing  more  and 
more  extensive.  From  a  knowledge  of  Faraday's  law  and 
the  electrochemical  equivalent,  it  is  possible  to  figure  just 
how  much  electric  current  will  be  required  to  carry  out  any 
desired  electrochemical  process,  and  the  practical  electro- 
chemist  makes  constant  use  of  this  law  in  his  calculations. 

SUMMARY 

Electrolytic  conduction  is  always  accompanied  by  a  chemical  decom- 
position. In  the  simplest  case  of  electrolysis,  the  ions  of  the 
dissolved  substance  discharge  and  the  two  constituents  ap- 
pear at  the  electrodes.  Thus  the  electrolysis  of  hydrochloric 
acid  gives  hydrogen  and  chlorine,  respectively. 

Secondary  Reactions:  In  many  cases,  the  discharged  ions  yield 
chemically  active  elements  or  radicals  which  enter  into  a 
secondary  reaction  with  the  solution  so  that  the  products 
actually  liberated  do  not  correspond  with  the  ions  that  take 
part  in  the  conduction  through  the  liquid. 

Electroplating :  In  the  electrolysis  of  the  salts  of  the  less  active  metals, 
the  metals  themselves  are  deposited,  or  plated,  on  the  negative 
electrode.  In  this  wise  the  extensive  art  of  electroplating  is 
carried  out.  To  avoid  depletion  of  the  metal  in  the  solution, 
the  positive  electrode  is  made  of  a  heavy  mass  of  the  metal  to 
be  plated,  and  the  metal  then  dissolves  from  this  electrode  as 
fast  as  it  is  deposited  upon  the  other. 

Faraday's  Law:  The  passage  of  the  same  amount  of  electricity 
causes  the  liberation  of  chemically  equivalent  amounts  of  dif- 
ferent substances  at  the  electrodes.  The  electricity  required 
to  set  free  one  gram-atomic  weight  of  any  element  whose  ions 
carry  a  single  electron,  will  set  free  one  half  gram-atomic 
weight  of  any  element,  or  radical,  whose  ions  carry  two  elec- 
trons. 


326  ELECTROLYSIS 

Questions 

1.  How  does  electrolytic  conduction  differ  from  that  in  a  metallic 
conductor  ? 

2.  What  becomes  of  the  charges  which  the  ions  carry  up  to  the 
electrodes  ? 

3.  What  would  happen  if  a  dilute  solution  of  H2S04  were  to  be 
electrolyzed  for  a  long  time  ? 

4.  What  becomes  of  the  sulphate  radical  when  H2S04  is  electro- 
lyzed? 

6.  What  becomes  of  the  metallic  sodium  which  one  might  expect 
to  get  at  the  negative  electrode  when  sodium  hydroxide  solution 
is  electrolyzed? 

6.  What  facts  prove  that  there  is  a  greater  concentration  of  sul- 
phate ions  around  the  positive  electrode  than  anywhere  else  in  the 
solution  during  the  electrolysis  of  H2S04? 

7.  How  does  it  happen  that  the  acid  formed  around  the  positive 
electrode  during  the  electrolysis  of  sodium  sulphate  solution  exactly 
neutralizes  the  base  formed  around  the  negative  electrode? 

8.  What  sort  of  metals  would  be  obtained  in  the  metallic  state  on 
the  negative  electrode  during  the  electrolysis  of  their  salts  ? 

9.  Show   how   practical   use  is  made  of  the   above  type  of 
electrolysis. 

10.  Try  to  devise  a  means  of  measuring  electricity  based  upon 
what  you  have  just  learned  about  Faraday's  law. 

11.  What  weight   of   silver  would  be   deposited  from  AgN03 
solution  by  the  same  current  that  deposits  6.36  grams  of  copper  from 
Cu(N03)2  solution? 

12.  A  certain  amount  of  electric  current  liberates  27.1  kilograms 
of  aluminium  from  the  solution  of  aluminium  oxide  in  fused  cryolite 
(page  260).     How  much  copper  would  the  same  current  deposit  on 
the  negative  electrode  in  copper  refining  (page  254)  ? 


CHAPTER  XXVII 

THE    ELECTROMOTIVE   SERIES 

345.  Displacement  of  Hydrogen  by  Metals.  It  has  al- 
ready been  seen  in  the  chapters  on  the  Ionic  Theory  and  on 
Electrolysis  that  the  peculiar  properties  of  acid  hydrogen  are 
in  all  probability  due  to  the  latter  being  in  an  ionized  condi- 
tion and  carrying  positive  electric  charges. 

When  an  active  metal,  such  as  aluminium,  magnesium,  or 
sodium,  reacts  with  an  acid  and  sets  free  hydrogen  gas,  a 
salt  of  the  metal  is  formed  in  the  solution  and  we  know  that 
this  salt  is  ionized ;  we  also  know  that  the  hydrogen  as  gas 
is  no  longer  ionized,  for  it  is  not  possessed  of  any  free  electric 
charge.  The  metal  must  then  have  taken  the  electric  charges 
away  from  the  hydrogen  ions,  and  the  hydrogen  atoms  with- 
out the  charges  have  combined  to  form  the  molecules,  H2, 
of  ordinary  hydrogen.  Thus  the  displacement  of  hydrogen 
by  metallic  sodium  may  be  represented  as  follows : 

Na  +  H+Cr  -+  Na+Cr  +  H. 

In  order  to  obtain  molecular  hydrogen,  this  equation  must 
be  taken  twice  so  as  to  give  two  atoms.  Since  the  chlorine 
ion  remains  unchanged,  it  may  be  omitted  from  both  sides 
in  writing  the  equation  and  thus  we  have 

2  Na  +  2  H+  ->  H2  +  2  Na+ . 

The  metal  on  acquiring  the  positive  charges  from  the  hydro- 
gen passes  into  the  form  of  ions  and  as  such  no  longer  exists 

327 


328  THE  ELECTROMOTIVE  SERIES 

as  a  compact  metallic  mass,  but  forms  a  component  of  the 
solution,  whereas  the  hydrogen  on  passing  out  of  the  ionic 
condition  ceases  to  be  a  component  of  the  solution. 

346.  Displacement  of  One  Metal  by  Another.  Not  only 
can  an  active  metal  displace  hydrogen  from  the  solution  of  an 
acid,  but  it  can  displace  metals  less  active  than  itself  from  the 
solutions  of  their  salts.  For  example,  when  a  knife  blade  is 
dipped  into  a  solution  of  copper  sulphate,  a  copper-colored 
coating  is  at  once  deposited  on  the  blade.  The  coating  is 
copper,  and  at  the  same  time  that  it  is  being  thrown  down, 
iron  from  the  knife  blade  is  passing  into  solution.  It  is  true 
that  the  passage  of  iron  into  the  solution  cannot  be  seen  as 
can  the  depositing  of  the  metallic  copper,  but  we  can  easily 
convince  ourselves  that  it  takes  place ;  for  if  after  a  little  while 
the  solution  is  tested  chemically,  it  is  found  to  contain  iron 
ions.  Iron  displaces  hydrogen  from  sulphuric  acid  and  in 
just  the  same  way  that  it  displaces  the  copper  from  copper 
sulphate,  except  that  free  un-ionized  copper  forms  a  compact 
metallic  mass  rather  than  a  gas.  The  atoms  of  iron  of  course 
acquire  electric  charges  and  pass  into  the  solution  as  ions, 
while  simultaneously  the  copper  ions  lose  their  charges  and 
are  deposited  as  metallic  copper  : 

Fe  +  Cu++SO4--->Fe++S<V     +  Cu, 
or,  since  the  sulphate  ion  is  unchanged, 

Fe  +  Cu++  ->  Fe++  +  Cu. 

A  number  of  active  metals  besides  iron  can  displace  copper 
and  hydrogen  from  solutions.  Among  them  are  the  ex- 
tremely active  metals  sodium  and  potassium,  which  react 
with  great  violence,  the  active  metals  magnesium,  aluminium, 
and  zinc,  which  react  rapidly,  and  the  moderately  active 
metals  such  as  tin  and  lead,  which  react  rather  slowly.  Not 


ELECTROMOTIVE  SERIES 


329 


only  do  these  metals  displace  hydrogen  and  copper,  but  they 
displace  the  precious  metals,  silver,  gold,  and  platinum,  from 
solutions  of  their  salts.  In  fact,  copper  itself  displaces  the 
metals  silver,  gold,  and  platinum. 

If  a  little  piece  of  clean  copper  is  placed  in  some  silver  nitrate 
solution  in  a  test  tube,  a  moss-like  deposit  begins  at  once  to 
form  over  the  surface  of  the  metal.  The  deposit  is  metallic 
silver,  and  it  appears  mossy  because  it  is  built  up  of  tiny 
branching  crystals  instead  of  forming  a  smooth  uniform  layer. 
At  the  same  time  the  solution  acquires  a  blue  color  which  is 
due  to  the  blue  copper  ions  that  are  formed. 

Cu  +  2  Ag+  -*  Cu++  +  2  Ag. 

Copper  ions  always  possess  the  same  blue  color  whatever  the 
negative  ions  that  balance  them.  Thus  dilute  solutions  of 
copper  nitrate,  copper  sulphate,  and  copper  chloride  all  pos- 
sess exactly  the  same  color. 

347.  Electromotive     Series.      All     metals 
seem  to  possess  a  tendency  to  pass  into  the 
form  of  ions  when  in  contact  with  water  or 
with  a  solution.     The  magnitude  of  this  tend- 
ency is  enormous  with  metals  like  sodium 
and  potassium ;  it  is  less,  although  large,  with 
magnesium,  aluminium,  and  zinc;  it  is  small 
with  iron,  tin,  and  lead ;    and,  finally,  it  is 
very  small   with  the  precious  metals.    'The 
metals  can  all  be  arranged,  as  in  the  table,  in 
the  order  in  which  they  possess  this  tendency, 
and  the  series  so  obtained  is  known  as  the 
electromotive  series. 

348.  Hydrogen  is  given  a  position  in  this 
series  because  it  is  like  the  metals  in  that  it 


Potassium 

Sodium 

Calcium 

Magnesium 

Aluminium 

Zinc 

Iron 

Nickel 

Tin 

Lead 

Hydrogen 

Copper 

Mercury 

Silver 

Platinum 

Gold 


Electromotive 
Series. 


330  THE  ELECTROMOTIVE  SERIES 

forms  simple,  positive  ions.  Any  metal  in  this  series  can 
drive  any  metal  standing  below  it  from  the  ionic  into  the 
metallic  condition,  whereas  its  own  ions  can  be  driven  out 
of  solution  by  any  metal  standing  above  it.  Thus  all 
the  metals  standing  above  hydrogen  cause  the  evolution 
of  hydrogen  gas,  when  placed  in  an  acid  solution,  but  the 
metals  copper,  silver,  mercury,  gold,  and  platinum,  which 
stand  below,  do  not  displace  hydrogen. 

349.  Sodium  reacts  violently  even  with  pure  water  and 
liberates  hydrogen.     Water  itself  ionizes  to  an  infinitesimal 
extent,  although  so  little  that  this  point  has  been  ignored 
in  all  our  previous  discussions.     Sodium  is  so  very  active 
that  it  reacts  even  with  the  few  hydrogen  ions  in  water 
and  drives  them  out.     This  quantity  would  be  altogether 
too  small  to  be  perceptible,  except  that,  with  these  ions 
removed,  the  water  dissociates  further  so  as  to   keep   up 
its  normal  infinitesimal  supply  of  ions.     The  sodium  con- 
tinues to  react   with   these  few   ions,    and    new   ones   are 
continually  formed  in  their   place,  so  that  in  the  end  the 
sodium  has  all  reacted  and  liberated  an  equivalent  amount 
of  hydrogen  from  the  water  which  at  the  outset  was  prac- 
tically un-ionized. 

350.  The  farther  above  hydrogen  a  metal  stands  in  the  elec- 
tromotive series,  the  greater  is  its  tendency  to  pass  into  the 
form  of  ions,  and  consequently  the  more  rapidly  should  it  be 
able  to  displace  hydrogen.     By  testing  all  of  the  metals  with 
an  acid,  for  example  dilute  hydrochloric  acid,  one  finds  that 
the  metals  towards  the  top  of  the  list  cause  a  violent  evolu- 
tion of  hydrogen  ;  that  those  lower  down  react  less  vigorously ; 
that  tin  and  lead,  which  stand  only  just  above  hydrogen,  dis- 
place hydrogen,  but  so  slowly  that  it  might  escape  observa- 
tion unless  carefully  looked  for;    and  that  all  of  the  metals 


SUMMARY  331 

below  hydrogen  in  the  list  cause  no  evolution  at  all  of  hydro- 
gen gas. 

351.  Electrochemical  Equivalents.  In  the  chapter  on 
Electrolysis,  it  was  found  to  be  a  law  that  the  passage  of  the 
same  amount  of  electricity  causes  the  liberation  of  equivalent 
amounts  of  different  substances  at  the  electrodes.  The  law, 
which  we  know  as  Faraday's  law,  applies  equally  well  to  the 
displacement  of  one  metal  by  another  metal,  and  in  reality 
the  latter  sort  of  displacement  is  just  as  much  an  electrolytic 
action  as  if  the  current  entered  by  one  electrode  and  left  by 
another  electrode.  Here  the  same  piece  of  metal  serves  as 
both  electrodes  in  short  electric  circuits.  At  some  points 
on  the  surface,  its  own  atoms  are  passing  into  the  solution 
as  ions  and  taking  positive  charges  with  them,  while  at  other 
points,  the  ions  of  hydrogen  or  other  metal  are  depositing 
out  of  the  solution  and  returning  just  as  much  positive 
electricity  to  the  piece  of  metal  as  was  given  off  at  the  first 
mentioned  points.  Hence  the  quantities  of  the  different 
metals  involved  in  these  displacements  are  chemically  equiv- 
alent quantities. 

SUMMARY 

Electromotive  Series :  All  metals,  in  which  classification  hydrogen 
is  included,  have  the  ability,  under  proper  conditions,  to  pass 
back  and  forth  between  the  forms  of  ordinary  massive  metals, 
and  positively  electrified  ions  in  solutions  of  the  metal  salts 
or  of  acids.  The  metals  are  arranged  in  a  series  in  the  order 
of  their  tendency  to  pass  into  the  ionic  form.  This  is  known 
as  the  Electromotive  Series. 

Metals  standing  above  hydrogen  in  the  electromotive  series  set  free 
hydrogen  from  acids ;  those  standing  below  hydrogen  cannot 
set  hydrogen  free. 

As  a  general  rule,  a  metal  can  drive  any  metal  standing  below  it 
in  the  electromotive  series  from  the  ionic  into  the  metallic 


332  THE  ELECTROMOTIVE  SERIES 

form.     The  amounts  of  the  metals  involved  in  such  displace- 
ments are  always  equivalent. 

Questions 

1.  Explain  the  difference  between  copper  in  the  metallic  condi- 
tion and  in  solution  —  using  the  ionic  theory  in  your  explanation. 

2.  Write,  in  ionic  form,  the  equation  for  the  displacement  of  copper 
by  iron.    Assume  that  the  copper  was  present  as  dissolved  copper 
sulphate. 

3.  Between  what  two  members  of  the  electromotive  series  should 
we  draw  a  line  to  separate  those  metals  which  can  displace  hydrogen 
from  acid  solution  from  those  that  cannot  f 

4.  Why  should  hydrogen  be  included  in  a  list  of  metals? 

5.  How  is  it  possible  that  the  active  metals  displace  hydrogen 
from  water? 

6.  According  to  Faraday's  law  —  as  applied  to  the  displacement 
of  one  metal  by  another  — •  what  weight  of  metallic  copper  would 
be  displaced  by  56  grams  of  iron?    By  65  grams  of  zinc?     By  24 
grams  of  magnesium?    By  27  grams  of  aluminium ? 

7.  Write  in  ionic  form  equations  for  the  following : 

(a)  Zn  +  HC1  (6)  Zn  +  CuCl2  (c)  Zn  +  AgNO3 

(d)  Al  +  Hg(N03)2  (e)   Fe  +  AuCl3  (/)  Cu  +  ZnCl2 

(g)  Cu  +  HC1  (h)  Cu  +  HgN03  (i)  Cu  +  Hg(NO3)2 

0')  Cu  +  PtCU  (*)  Ag  +  AuCl3  (/)  Ag  +  PtCl4 


CHAPTER  XXVIII 
HYDROGEN  EQUIVALENTS  AND  VALENCE 

352.  It  is  of  the  utmost  importance  in  chemistry  to  know 
what  amounts  of  different  elements  can  mutually  displace 
each  other  or  combine  with  one  another.     If  these  amounts 
are  always  referred  to  the  same  standard,  they  are  known 
as  equivalent  weights,  and  for  the  standard  of  reference,  one 
gram  atomic  weight,  that  is  practically  one  gram  of  hydro- 
gen, is  used. 

The  hydrogen  equivalent  of  any  metal  is  that  weight  of  the 
metal  which  is  required  to  displace  one  gram  of  hydrogen,  or, 
in  the  case  of  a  metal  standing  below  hydrogen  in  the  elec- 
tromotive series,  it  is  that  weight  of  the  metal  which  will  be 
deposited  by  the  same  quantity  of  electricity  as  will  set  free  one 
gram  of  hydrogen. 

353.  Experimental  Determination  of  Hydrogen  Equiva- 
lents.    Let  us  consider  in  some  detail  the  actual  steps  we 
would  follow  to  determine  the  hydrogen  equivalents  of  the 
three  common  metals,  aluminium,  magnesium,  and  sodium; 
and  then  let  us  study  the  results  and  see  how  they  compare 
with  the  atomic  weights  of  these  metals. 

To  find  the  weight  of  aluminium  which  will  displace 
one  gram  of  hydrogen  from  an  acid,  we  take  a  little  piece  of 
pure  aluminium  wire  and  weigh  it  accurately.  It  weighs 
0.0388  gram  in  a  specific  case  that  we  are  considering.  The 
wire  is  placed  in  the  bottom  of  a  good-sized  beaker  and  over 
it  is  placed  an  inverted  funnel  to  direct  the  hydrogen  gas 

333 


334 


HYDROGEN  EQUIVALENTS  AND   VALENCE 


into  the  collecting  vessel   (Fig.   67).     The  beaker  is  now 

filled  with  water  until  the  stem  of  the  funnel  is  covered. 

To  measure  the  hydrogen  gas,  we  take  a  100  c.c.  measuring 
tube  such  as  that  used  for  determining  the 
volume  per  cent  of  oxygen  in  air  (page  74) . 
This  tube  is  filled  completely  with  hydro- 
chloric acid,  the  thumb  is  placed  over  the 
mouth  of  the  tube,  and  the  latter  is  then 
inverted  over  the  stem  of  the  funnel  in 
the  beaker.  When  the  thumb  is  removed, 
the  heavier  acid  sinks  through  the  water 
to  the  bottom  of  the  beaker  where  it 
comes  in  contact  with  the  aluminium  and 
reacts  with  it.  The  hydrogen  evolved 
rises  through  the  stem  of  the  funnel  and 
into  the  measuring 'tube.  When  all  the 
aluminium  has  reacted,  the  tube  is  some- 
what more  than  one  half  filled  with  hy- 
drogen, the  volume  of  which  is  now  to 
be  accurately  measured.  The  thumb  is 
FIG  67  —  Deter-  agam  placed  over  the  mouth  of  the  tube, 

mining  the   Hydrogen   and  without  allowing  any  gas  to  escape  or 

Equivalent  of  a  Metal.      &{jt  t()  ^^  ^    tube    jg    transferred    to    a 

deep  vessel  of  water  where  its  height  is  so  adjusted  that  the 
water  level  inside  the  tube  is  the  same  as  that  outside. 
The  volume  is  then  read  off  from  the  markings  on  the  tube 
and  at  the  same  time  the  temperature  and  the  barometric 
pressure  are  noted.  The  data  thus  obtained  are  : 

Weight  of  aluminium 0.0384  g. 

Measured  volume  of  hydrogen      ....  53.4  c.c. 

Temperature 20°  C. 

Barometric  pressure 740  mm. 


EXPERIMENTAL  DETERMINATION  335 

We  know  that  one  liter  of  hydrogen  under  standard  con- 
ditions weighs  0.09  gram,  but  before  we  can  calculate 
the  weight  of  the  gas  obtained,  we  must  find  its  volume  under 
standard  conditions. 

Applying  the  correction  for  temperature: 


53.4  c.c.  at  20°  =  53.4  X         =  49.8  c.c.  at  0° 
and  for  pressure  : 

m.  =  49.8 

760 


74-0 
49.8  c.c.  at  740  mm.  =  49.8  X  -      =  48.5  c.c.  at  760  mm.1 


This  volume  weighs  : 

—  X  0.09  =  0.00427  gram. 
1000 

We  have  now  found  that  0.0384  gram  of  aluminium  dis- 
places 0.00427  gram  of  hydrogen.  Therefore  x  grams  of 
aluminium  will  displace  1  gram  of  hydrogen. 

0.0384  =  0.00427 

x  1 

0.0384 


and  the  hydrogen  equivalent  of  aluminium  is  9. 

Experiments  carried  out  with  magnesium  and  sodium  2 
after  the  manner  just  described  for  aluminium  show  that  12 
grams  of  magnesium  are  required  to  displace  1  gram  of  hydro- 
gen and  that  23  grams  of  sodium  are  required  to  displace 
1  gram  of  hydrogen.  The  hydrogen  equivalent  of  magne- 
sium is  therefore  12,  and  that  of  sodium  is  23. 

354.   The  atomic  weights  of  these  three  elements  are, 

1  This  is  only  approximate,  since  water  vapor  is  neglected. 

2  Sodium  is  so  active  that  the  method  must  be  modified  in  some 
of  its  details. 

B.  AND  W.  CHEM.  -  22 


336  HYDROGEN  EQUIVALENTS  AND   VALENCE 

aluminium  27,  magnesium  24,  and  sodium  23.  So  it  is  seen 
that  the  hydrogen  equivalent  and  the  atomic  weight  are  iden- 
tical in  the  case  of  sodium ;  that  the  hydrogen  equivalent  of 
magnesium  is  one  half  the  atomic  weight ;  and  that  the  hy- 
drogen equivalent  of  aluminium  is  one  third  of  the  atomic 
weight.  From  this  it  follows  that  one  atomic  weight  of 
sodium  can  displace  a  single  atomic  weight  of  hydrogen ; 
that  one  atomic  weight  of  magnesium  can  displace  two  atomic 
weights  of  hydrogen ;  and  that  one  atomic  weight  of  alumin- 
ium can  displace  three  atomic  weights  of  hydrogen. 

355.  Valence.     The   number   which   expresses   how   many 
atomic  weights  of  hydrogen  are  displaced  by  one  atomic  weight 
of  an  element  is  the  valence  of  that  element  (see  section  215). 

Thus,  since  one  atomic  weight  of  aluminium  can  take  the 
place  of  three  atomic  weights  of  hydrogen  when  this  metal 
reacts  with  an  acid, 

Al  +3HC1->A1C18  +  3H, 

the  valence  of  aluminium  is  three.  Similarly  the  valence 
of  magnesium  is  two,  and  that  of  sodium  is  one. 

356.  It  has  already  been  seen  that  hydrogen  is  more  like 
the  metals  than  the  non-metals  in  its  chemical  properties. 
It  forms  positive  ions  like  the  metals,  never  negative  ions 
as  do  the  non-metals.     Its  shows  little  ability  to  form  com- 
pounds with  the  metals,  to  which  it  is  similar  in  nature; 
but  it  does  form  compounds  with  the  non-metals,  to  which 
it  is  dissimilar. 

Hydrogen  can  take  the  place  of,  or  be  displaced  by,  metals, 
and  the  valence  of  a  metal  is  measured  by  the  amount  of 
hydrogen  to  which  it  is  equivalent.  But  the  valence  of  a 
non-metallic  element  cannot  be  measured  by  the  number  of 
atomic  weights  of  hydrogen  that  one  atomic  weight  can 


VALENCE  337 

displace,  because  hydrogen  and  the  non-metals,  being  oppo- 
site in  their  chemical  nature,  are  not  mutually  interchange- 
able. But  non-metals  can  combine  with  hydrogen,  and  their 
valence  is  measured  by  the  number  of  atomic  weights  of  hy- 
drogen which  one  atomic  weight  of  the  element  will  hold 
in  chemical  combination. 

The  weight  of  chlorine  which  combines  with  1  gram  of 
hydrogen  to  form  hydrogen  chloride  is  35.5  grams,  and  since 
this  quantity  is  the  gram-atomic  weight  of  chlorine,  it 
follows  that  chlorine  combines  with  hydrogen,  atomic 
weight  for  atomic  weight,  therefore  the  valence  of  chlorine 
is  one. 

The  weight  of  oxygen  which  combines  with  1  gram  of 
hydrogen  to  form  water  is  8  grams.  Since  this  is  one  half 
the  gram-atomic  weight  of  oxygen,  it  follows  that  one  whole 
atomic  weight  of  oxygen  can  combine  with  two  atomic 
weights  of  hydrogen;  therefore  the  valence  of  oxygen  is 
two. 

The  weight  of  sulphur  which  combines  with  1  gram  of 
hydrogen  is  16  grams.  The  atomic  weight  of  sulphur  is  32 ; 
hence  16  grams  is  one  half  of  a  gram-atomic  weight.  One 
whole  atomic  weight  of  sulphur  combines  with  two  atomic 
weights  of  hydrogen  and  the  valence  of  sulphur  is  two. 

357.  When  it  is  desired  to  know  the  valence  of  any  ele- 
ment, it  is  only  necessary  to  find  the  composition  of  its  simple 
compounds.  The  knowledge  of  the  composition  of  all  the 
compounds  that  have  been  thoroughly  studied  is  already 
at  hand,  and  this  knowledge  is  expressed  in  the  chemical 
formulas  of  the  compounds.  Hence  to  find  the  valence 
of  an  element,  we  refer  to  the  formulas  of  its  simple  com- 
pounds. 

The  formula  of  hydrogen  chloride,  HC1,  tells  us  that  chlo- 


338  HYDROGEN   EQUIVALENTS   AND   VALENCE 

rine  combines  atomic  weight  for  atomic  weight,  or  atom  for 
atom,  with  hydrogen  and  that  the  valence  of  chlorine  is  one. 
The  formulas  of  water  and  hydrogen  sulphide  are  H2O  and 
H2S,  respectively,  and  the  valence  of  oxygen,  as  well  as  of 
sulphur,  is  therefore  two. 

358.  In  compounds  containing  no  hydrogen,  comparison 
must  be  made  with  the  known  valence  of  one  of  the  elements 
of  the  compound.     In  the  compound  potassium  chloride, 
whose  formula  is  KC1,  the  valence  of  chlorine  is  known  to  be 
one,  because  chlorine  has  already  been  compared  directly 
with  hydrogen  as  stated  above.     Then  since  potassium  com- 
bines with  chlorine  in  the  proportion  of  atom  for  atom,  the 
valence  of  potassium  must  also  be  one.     That  this  method 
of  comparison  is  justified,  may  be  shown  from  the  hydro- 
gen  equivalent   of   potassium.     It   requires    39    grams    of 
potassium,  or  one  gram-atomic  weight,  to  displace  1  gram 
of  hydrogen. 

The  valence  of  magnesium  is  seen  to  be  two  by  an  inspec- 
tion of  the  formula  of  magnesium  chloride,  MgCl2 ;  and  the 
valence  of  aluminium  to  be  three  from  the  formula  Aids. 

359.  Valence  in  Compounds  Containing  More  than  Two 
Elements.     When  three  or  more  elements  are  present  in  a 
compound,  the  problem  of  finding  the  valence  of  each  ele- 
ment becomes  more  complicated.     More  must  be  known 
about  the  mutual  relations  of  the  elements  in  the  compound 
than  is  shown  in  the  formula.     Indeed,  very  different  opin- 
ions are  held  to-day  by  the  foremost  chemists  as  to  the  real 
valence  of  some  of  the  elements  in  complex  compounds  ;  and, 
such  being  the  case,  we  shall  not  attempt  here  to  go  into  the 
question.     We  shall  satisfy  ourselves  with  applying  our  ideas 
of  valence  to  the  compounds  containing  two  elements  only, 
and  within  these  limits  we  shall  find  the  conception  of  valence 


VARIABLE  VALENCE  339 

to  be  of  the  greatest  service  in  understanding  chemical  com- 
binations. 

360.  Variable    Valence.     Many    of    the    elements    show 
different  valences  according  to  different  conditions  under 
which  they  combine  with  other  elements.     For  example, 
sulphur  may  combine  with  either  two  or  three  atomic  weights 
of  oxygen  per  atomic  weight  of  sulphur  (see  page  350).     The 
valence  of  sulphur  in  sulphur  dioxide,  SO2,  is  four  because  the 
sulphur  combines  with  two  oxygens  each  of  which  has  a  va- 
lence of  two  against  hydrogen.     In  sulphur  trioxide,  SOs,  the 
valence  of  sulphur  is  six. 

Many  other  cases  of  change  of  valence  are  known  and 
in  the  table  on  page  340  is  found  a  list  of  the  commoner  ele- 
ments with  their  valences ;  in  many  instances  it  is  seen  that 
two  or  more  valences  are  given  opposite  a  single  element. 

361.  The  valence  which  an  element  shows  toward  hydro- 
gen is  very  often  different  from  the  valence  of  the  same  ele- 
ment towards  oxygen.     Thus  sulphur,  which  we  have  just 
seen  has  the  valence  of  four  or  six  towards  oxygen,  shows  the 
valence  of  two  against  hydrogen  in  hydrogen  sulphide,  H2S. 
A  considerable  degree  of  regularity  in  the  valence  of  elements 
of  different  families  as  well  as  in  the  differences  in  the  valences 
displayed  towards  hydrogen  and  oxygen,  is  shown  in  the 
periodic  arrangement  of  the  elements  on  the  page  facing  the 
inside  back  cover. 

362.  As  already  hinted,  the  conception  of  valence  and  a 
knowledge  of  valence  numbers  is  an  invaluable  aid  in  de- 
veloping our  knowledge  of  chemistry.     For  example,  if  we 
know  the  combining  habits  and  the  valence  numbers  of  all 
the  elements,  we  can  predict  with  certainty  how  any  two 
elements  will  combine  in  a  compound  of  which  we  have  no 
other  knowledge. 


340 


HYDROGEN  EQUIVALENTS   AND   VALENCE 


TABLE  OF  THE  COMMONER  ELEMENTS,  THEIR  SYMBOLS,  VALENCES, 
AND  APPROXIMATE  ATOMIC  WEIGHTS 


NAME 

SYMBOL 

ATOMIC 

WEIGHT 

VALENCE 

Towards 
Hydrogen  or 
Metals 

Towards  Oxygen 
or  Chlorine 

Aluminium 

Al 

27 

0 

III 

Antimony 

Sb 

120 

III 

III,  V 

Argon 

A 

40 

0 

0 

Arsenic 

As 

75 

III 

III,  V 

Barium 

Ba 

137 



II 

Bismuth 

Bi 

208 

0 

III 

Boron 

B 

11 



III 

Bromine 

Br 

80 

I 

I,  III,  V,  VII 

Cadmium 

Cd 

112 

0 

II 

Calcium 

Ca 

40 



11 

Carbon 

C 

12 

IV 

IV 

Chlorine 

Cl 

35.5 

I 

I,  III,  V,  VII 

Copper 

Cu 

63.6 

—  - 

I,  II 

Fluorine 

F 

19 

1 

0 

Gold 

Au 

197 

0 

I,  III 

Helium 

He 

4 

0 

0 

Hydrogen 

H 

1 



I 

Iodine 

I 

127 

I 

I,  III,  V,  VII 

Iron 

Fe 

56 

0 

II,  III 

Lead 

Pb 

207 

0 

II,  IV 

Lithium 

Li 

7 



1 

Magnesium 
Manganese 

Mg 
Mn 

24.3 
55 

0 

11 

11,  III,  IV,  VI,  VII 

Mercury 

Hg 

200 

0 

1,11 

Nickel 

Ni 

58.7 

0 

II,  III 

Nitrogen 

N 

14 

III 

III,  V 

Oxygen 

0 

16 

II 



Phosphorus 

P 

31 

III 

Ill,  V 

Platinum 

Pt 

195 

0 

II,  IV 

Potassium 

K 

39 



I 

Silicon 

Si 

28.3 

IV 

IV 

Silver 

Ag 

108 

0 

I 

Sodium 

Na 

23 



I 

Strontium 

Sr 

88 



II 

Sulphur 

S 

32 

II 

IV,  VI 

Tin 

Sn 

119 

0 

II,  IV 

Zinc 

Zn 

65.4 

0 

11 

QUESTIONS  341 

SUMMARY 

The  hydrogen  equivalent  is  that  weight  of  a  metal  that  is  inter- 
changeable with  1  gram  of  hydrogen.  This  amount  of  the 
metal  is  equivalent  both  electrically  and  chemically  to  one 
gram-atomic  weight  of  hydrogen. 

The  valence  of  a  metal  is  the  number  of  gram-atomic  weights  of 
hydrogen  which  are  equivalent  to  one  gram-atomic  weight  of 
the  metal. 

The  valence  of  a  non-metal  is  the  number  of  gram-atomic  weights 
of  hydrogen  which  one  gram-atomic  weight  of  the  non-metal 
holds  in  combination. 

In  compounds  that  contain  no  hydrogen,  the  valence  of  the  elements 
is  derived  from  comparison  with  compounds  that  do  contain 
hydrogen. 

The  valence  of  an  element  in  a  simple  compound  is  usually  apparent 
from  an  inspection  of  the  formula  of  the  compound.  The 
conception  of  valence  is  often  difficult  to  apply  to  compounds 
that  contain  more  than  two  elements. 

Variable  Valences :  The  same  element  frequently  displays  different 
valences  under  different  conditions.  The  valence  of  an  ele- 
ment towards  hydrogen  is  often  different  from  th$  valence 
towards  oxygen. 

The  conception  of  valence  is  of  great  value  in  developing  our  knowl- 
edge of  chemistry. 

Questions 

1.  Define  "  hydrogen  equivalent." 

2.  Explain  how  the  weight  of  the  hydrogen  displaced  by  a  known 
weight  of  a  metal  may  be  found  without  actually  weighing  the  gas. 

3.  Calculate  the  hydrogen  equivalent  of  cadmium  from  the  follow- 
ing data  :     0.1 12  g.  of  cadmium  released  24.4  c.c.  of  hydrogen  at  20° 
C.  and  740  mm.     (1  liter  H2  at  standard  conditions  =  .09  g.) 

4.  The  atomic  weight  of  cadmium  is  112.     Using  the  hydrogen 
equivalent  calculated  in  Question  3,  find  the  valence  of  cadmium. 

5.  Calculate  what  weight  of  zinc  (atomic  weight,  65  —  hydrogen 
equivalent,  32.5)  would  be  convenient  to  use  in  a  hydrogen  equiv- 
alent determination  —  the  gas  measuring  tube  having  a  volume 
of  100  c.c. 


CHAPTER  XXIX 

SULPHUR 

363.  Familiar  Forms  and  Uses  of  Sulphur.  Sulphur  is 
familiar  to  us  in  the  uncombined  state  as  a  brittle,  yellow, 
non-metallic  substance.  We  can  buy  it  at  a  drug  store  in 
the  form  of  a  yellow  powder  or  of  coarse  lumps  or  sticks. 
Powdered  sulphur  is  used  as  an  ingredient  of  various  salves 
on  account  of  its  germicidal  properties  and  it  is  often  given 
internally  for  the  purpose  of  "  purifying  the  blood."  A 
familiar  old-fashioned  remedy  for  the  blood,  that  very 
likely  has  a  good  deal  of  virtue,  is  a  mixture  of  sulphur  and 
molasses.  Sulphur  is  boiled  with  lime  and  water  to  make 
"  lime  sulphur  "  spray  for  killing  insect  pests  that  destroy 
our  fruit  trees.  Sticks  of  sulphur,  or  prepared  sulphur 
candles,  are  burned  in  rooms  after  sickness  in  order  to  de- 
stroy lingering  disease  germs.  Sulphur  is  likewise  burned 
in  clothes  closets  in  order  to  kill  moths.  The  suffocating 
fumes  of  burning  sulphur,  which  are  composed  of  sulphur 
dioxide,  are  familiar,  and  in  view  of  their  offensiveness,  it  is 
not  surprising  that  they  are  powerful  in  destroying  pests. 
Sulphur  is  used  in  large  quantities  to  "  vulcanize  "  rubber. 

Sulphur  under  ordinary  conditions  is  a  crystalline  solid. 
The  masses  which  we  more  usually  see  are  opaque  because 
they  are  composed  of  a  great  number  of  very  small  crystals 
overlapping  and  interlacing.  It  is  well  known  that  if  the 
clearest  specimen  of  transparent  glass  is  pulverized,  the 
powdered  mass  thus  obtained  presents  the  opaque  appear- 

342 


SULPHUR 


343 


ance  of  snow.     Thus  with  the  mass  of  small  sulphur  crystals, 
each  one  of  which  by  itself  is  transparent. 

When  sulphur  crystallizes  under  the  right  conditions, 
that  is,  when  the  crystallization  takes  place  very  slowly,  as 
for  example  in  fissures 
in  the  earth,  clear 
transparent  crystals 
of  considerable  size 
are  formed.  These 
crystals  are  very 
beautiful ;  they  are 
bounded  by  smooth 
lustrous  surfaces 
which  lie  in  certain 


definite     planes    with 
reference  to  imaginary 


FIG.  68.  —  Rhombic  Sulphur  Crystals,  clinging  to 


axes    of    the    cr\TStals.     a  fragment  of  rock  broken  from  the  interior  of  a 
cavity  in  the  earth. 

Some    idea    of    their 

shape  and  appearance  may  be  obtained  from  Fig.  68.  It  is 
easy  to  obtain  very  small  crystals  of  this  kind  in  the  labora- 
tory, and  they  are  just  as  perfect  in  shape  as  the  large  ones. 
It  is  only  necessary  to  dissolve  some  sulphur  in  carbon 
disulphide,  a  liquid  in  which  it  is  very  soluble,  and  to  leave 
the  solution  in  a  watch  glass  for  a  little  time  while  the 
carbon  disulphide  evaporates.  A  number  of  the  small  crys- 
tals are  left  in  the  watch  glass  when  the  liquid  has  gone. 
These  crystals  belong  to  a  certain  system  of  crystals  known 
as  the  rhombic  system,  and  they  are  therefore  spoken  of  as 
rhombic  sulphur  crystals. 

364.  Allotropic  Forms  of  Sulphur.  It  is  well  known  that 
uncombined  carbon  can  exist  in  different  forms,  as  char- 
coal, graphite,  and  diamond  (Chapter  XXIII).  Charcoal  is 


344 


SULPHUR 


amorphous,  that  is,  without  definite  shape,  whereas  the  other 
two  forms  are  crystalline  as  they  occur  in  nature.  Sulphur 
likewise  is  known  in  three  different  forms,  although  these 
forms  do  not  differ  so  extremely  from  one  another  as  do 
graphite  and  diamond.  The  rhombic  crystals  which  have 
just  been  described  are  the  most  frequent  of  the  three  forms. 

Another  crystal- 
line variety  of 
sulphur  can  be  ob- 
tained if  a  mass  of 
melted  sulphur  is 
allowed  to  cool 
and  solidify  slowly. 
These  crystals  ap- 
pear in  the  shape  of 
long  needles,  which 
belong  to  the  so- 
called  monoclinic  system,  and  are  known  as  monoclinic 
sulphur.  They  can  be  obtained  in  the  laboratory  if  sulphur 
is  melted  in  a  clay  crucible  and  is  then  left  undisturbed  to 
cool.  When  a  thin  solid  crust  has  just  formed  over  the 
surface,  if  a  hole  is  broken  and  the  still  liquid  sulphur  un- 
derneath is  poured  out,  a  loose  mass  of  needle-like  crys- 
tals is  found  to  fill  the  interior  of  the  crucible.  An  idea  of 
their  appearance  can  be  obtained  from  Fig.  69. 

Another  form  of  sulphur,  the  so-called  amorphous  form, 
is  obtained  when  melted  sulphur,  which  has  been  heated  to 
about  250°  C.,  is  suddenly  chilled  as  when  it  is  poured  into 
water.  At  first  this  cooled  sulphur  is  of  a  consistency  much 
like  soft  rubber,  but  on  standing  it  gradually  stiffens  and 
becomes  brittle  like  ordinary  sulphur. 
The  common  form  of  sulphur  melts  at  114.5°  C.  to  a  pale 


FIG.  69.  —  Monoclinic  Sulphur  Crystals. 


OCCURRENCE  OF  SULPHUR 


345 


yellow,  mobile  liquid.  Liquid  sulphur  has  one  very  sur- 
prising property,  namely,  when  heated  to  a  higher  tempera- 
ture, instead  of  growing  even  more  mobile,  as  do  liquid 
substances  generally,  it  becomes  viscous.  At  the  same  time 
it  turns  dark  in  color.  At  about  250°  C.  it  is  so  viscous  that 
the  vessel  containing  it  may  be  inverted  without  its  running 
out.  Above  250°  C.  it  again  grows  less  viscous  and  at  448°  C. 
it  boils  and  is  converted  into  a  pale  yellowish-brown  vapor. 
365.  Occurrence  of  Sulphur.  Sulphur  occurs  in  nature  as 
uncombined  sulphur;  as  sulphides  of  the  heavy  metals,  es- 
pecially of  iron,  copper,  lead,  and  zinc;  and  as  sulphates, 
calcium  sulphate  (gypsum  is  CaSO4  •  2  H2O),  barium  sul- 
phate, and  lead  sulphate. 


FIG.  70. —  A  Block  of  Louisiana  Sulphur.  The  molten  sulphur  is  pumped 
from  the  wells  into  huge  wooden  bins.  The  solid  block  in  the  picture  is  48 
feet  high.  The  boards  have  been  removed  from  the  front  face  of  the  bin 
but  have  been  left  at  the  end.  The  sulphur  is  broken  from  the  front  of  the 
block  with  blasting  powder  and  is  loaded  on  to  freight  cars  for  shipment. 


346  SULPHUR 

Uncombined  sulphur  is  found  in  many  parts  of  the  earth, 
notably  in  Sicily,  in  volcanic  regions  where  it  is  deposited 
from  the  volcanic  gases  in  fissures  in  the  rocks.  It  is  also 
found  in  some  places  where  its  formation  does  not  seem  to 
be  due  to  volcanic  action,  notably  in  Louisiana. 

In  Sicily  sulphur  is  extracted  from  the  rocks  by  melting, 
the  heat  usually  being  obtained  by  the  crude  method  of 
burning  a  part  of  the  sulphur  itself.  In  Louisiana,  where 
the  deposits  are  deep  beneath  the  surface,  a  novel  method  has 
been  invented  for  extracting  the  sulphur.  Pipes  are  driven 
to  the  sulphur-bearing  stratum  and  superheated  water  is 
forced  down.  The  sulphur  is  thus  melted ;  it  sinks  to  the 
bottom  of  the  well  and  is  pumped  to  the  surface  by  means 
of  compressed  air  and  run  into  huge  wooden  bins,  where  it 
is  allowed  to  solidify. 

SULPHIDES  AND  HYDROGEN  SULPHIDE 

366.  Sulphides.  In  its  chemical  properties,  sulphur  is 
in  many  respects  like  the  typical  non-metals,  oxygen  and 
chlorine,  that  we  have  already  studied.  It  combines  with 
metallic  elements  —  including  hydrogen  —  forming  sulphides 
just  as  oxygen  and  chlorine  form  oxides  or  chlorides.  For 
example,  when  iron  or  copper  or  zinc  filings  are  mixed  with 
powdered  sulphur  and  heated  in  a  test  tube  over  a  Bunsen 
burner,  a  reaction  soon  begins,  and  if  the  tube  is  then  re- 
moved from  the  flame  the  reaction  continues  under  the  in- 
fluence of  the  heat  which  it,  itself,  develops,  and  the  incan- 
descence spreads  through  every  part  of  the  mixture.  The 
substance  that  remains  is  neither  sulphur,  nor  the  metal, 
but  is  the  sulphide  of  the  metal. 

Sulphur  also  combines  directly  with  hydrogen  but  not 


HYDROGEN  SULPHIDE  347 

as  energetically  as  with  the  metals.  If  sulphur  is  placed 
in  a  hard  glass  tube  and  a  current  of  hydrogen  is  passed  over 
the  sulphur  while  the  tube  is  heated  with  a  Bunsen  flame, 
a  little  hydrogen  sulphide  is  formed,  but  the  reaction  is  not 
energetic  and  continues  to  take  place  only  so  long  as  external 
heat  is  being  supplied. 

367.  Hydrogen  sulphide,  H2S,  is  a  gas  which  has  a  very 
offensive  odor,  the  odor  of  rotten  eggs.     It  is  frequently 
formed  in  the  decomposition  of  organic  matter  containing 
sulphur,  and  thus  its  odor  is  apparent  not  only  in  eggs,  but 
in  sewage.     When  the  mud  of  mud  flats  near  where  sewage 
is  discharged  is  disturbed  with  a  stick,  the  bubbles  of  gas 
which  rise  have  a  strong  odor  of  hydrogen  sulphide. 

This  gas  is  extremely  poisonous  to  inhale.  Breathing  the 
undiluted  gas  will  render  a  man  unconscious  almost  in- 
stantly, and  it  will  prove  fatal  unless  he  is  immediately 
removed  to  the  open  air  and  restoratives  are  employed. 
Breathing  very  dilute  hydrogen  sulphide,  as  it  is  often  pres- 
ent in  the  air  of  the  laboratory,  is  offensive  and  may  make 
one  feel  rather  sick,  but  it  is  not  especially  dangerous. 

The  so-called  sulphur  water  of  sulphur  springs  is  really 
water  charged  with  hydrogen  sulphide  issuing  from  the  in- 
terior of  the  earth.  A  small  amount  of  hydrogen  sulphide 
taken  into  the  stomach  in  this  way  is  not  injurious  and  is 
in  some  cases  perhaps  even  beneficial. 

368.  Laboratory  Method  of  Preparing  Hydrogen  Sulphide. 
In  the  laboratory,  hydrogen  sulphide  is  made  by  allowing 
an  acid  to  react  with  iron  sulphide. 

FeS  +  2  HC1  -^  FeCl2  +  H2S. 

The  desired  product,  being  a  gas,  escapes  as  fast  as  it  is 
formed  and  can  be  led  as  desired  through  delivery  tubes. 


348  SULPHUR 

369.  Hydrogen  Sulphide  as  a  Precipitant.     One  of  the 
most  important  uses  of  hydrogen  sulphide  is  as  a  precipitant. 
If  the  gas  is  bubbled  into  the  solution  of  a  salt  of  copper, 
silver,  mercury,  lead,  tin,  or  of  any  one  of  several  other 
of  the  heavy  metals,  the  sulphide  of  the  metal  is  precipitated. 
Hydrogen   sulphide   is   only   slightly  dissociated   into  ions, 
H2S  ^>  H+  +  H+  +  S~  ~~,  but  since  the  sulphides   of   the 
heavy  metals  are  extremely  insoluble,  even  the  few  sulphide 
ions  from  the  hydrogen  sulphide  cannot  remain  in  presence 
of  the  metal  ions.     Therefore  metal  sulphide  precipitates, 
but  as  the  sulphide  ions  are  thus  taken  out  of  the  solution, 
more  hydrogen  sulphide  is  able  to  dissociate  and  this  con- 
tinues until  there  are  no  more  metal  ions  left  to  remove 
further  sulphide  ions. 

By  the  formation  or  non-formation  of  a  precipitate  when 
hydrogen  sulphide  is  passed  into  a  solution,  it  is  possible  to 
tell  whether  salts  of  the  heavy  metals  are  present,  and  further- 
more the  color  of  the  precipitate  gives  an  indication  as  to  what 
metal  is  present.  Thus  the  sulphides  of  silver,  lead,  mercury, 
bismuth,  and  tin  in  the  divalent  condition  precipitate  from 
an  acidified  solution  either  black  or  very  dark  brown;  the 
sulphides  of  cadmium,  arsenic,  and  tin  in  the  tetravalent 
condition,  yellow;  the  sulphide  of  antimony,  orange  red. 
The  sulphide  of  zinc  precipitates  white  from  a  very  weakly 
acid  or  an  alkaline  solution,  but  not  from  a  strongly  acid  one ; 
and  from  an  ammoniacal  but  not  from  an  acid  solution,  the 
sulphides  of  iron,  nickel,  and  cobalt  precipitate  black  and  the 
sulphide  of  manganese,  flesh-colored. 

370.  Hydrosulphuric  Acid.     A  solution  of  hydrogen  sul- 
phide is  faintly  acid,  —  sufficiently  so  to  barely  turn  blue 
litmus  to  red.     This  indicates  a  small  content  of  hydrogen 
ions  and  corresponds  to  the  low  degree  of  dissociation  re- 


OXIDES  OF  SULPHUR  349 

f erred  to  in  the  foregoing  paragraph.  If  it  is  wished  to  lay 
emphasis  on  the  fact  that  hydrogen  sulphide  is  an  acid, 
it  is  spoken  of  as  hydrosulphuric  acid  (see  page  221). 
It  is  almost  never  used  for  its  acid  properties,  however,  and 
hence  we  rarely  see  it  mentioned  by  this  name. 

On  the  other  hand,  hydrogen  chloride  is  a  strong  acid 
when  it  is  dissolved  in  water,  that  is,  its  solution  contains 
a  large  proportion  of  hydrogen  ions.  Its  acid  properties 
are  the  foremost  in  importance  and  we  find  it  most  commonly 
called  hydrochloric  acid. 

OXIDES  OF  SULPHUR 

371.  Sulphur  Dioxide  and  Sulphurous  Acid.  It  is  well 
known  that  sulphur  burns  readily  in  the  air  and  forms  a  very 
choking  gas.  This  gas  is  sulphur  dioxide,  SO2.  To  pre- 
pare sulphur  dioxide  free  from  nitrogen  and  excess  of  air,  the 
usual  laboratory  method  is  to  heat  together  concentrated  sul- 
phuric acid  and  copper  turnings.  Sulphuric  acid  contains 
the  oxide  SO3  in  combination  with  water.  The  trioxide  is 
reduced  by  copper  to  sulphur  dioxide,  which  escapes  as  the 
gas  desired.  The  copper  is  at  the  same  time  oxidized  to 
copper  oxide,  and  the  copper  oxide  being  a  basic  oxide  re- 
acts with  surplus  sulphuric  acid  to  form  the  salt  copper 
sulphate : 

H2S04  ->  H20  +  SO3 
S03  +  Cu  -+  CuO  +  SO2 1 
CuO  +  H2SO4  -»  H2O  +  CuSO4 
Cu  +  2  H2SO4  -*  2  H2O  -f  CuSO4  +  SO2  f 

Sulphur  dioxide  is  somewhat  soluble  in  water  and  the 
solution  is  acid.  It  is  supposed  that  a  definite  compound, 
sulphurous  acid,  is  formed  :  H2O  +  SO2  ->  H2SO3,  but  this 


350  SULPHUR 

compound  is  so  unstable  that  it  is  impossible  to  separate  it 
out  in  the  pure  state  from  the  water  in  which  it  is  dissolved. 
The  main  reason  for  believing  in  the  existence  of  this  definite 
acid  is  that  salts  of  the  acid  can  be  prepared.  For  example, 
the  acid  solution  can  be  neutralized  by  adding  sodium  hy- 
droxide ;  then,  on  evaporation  of  the  neutral  solution,  a  solid 
substance  is  left  which  on  analysis  is  shown  to  have  a  com- 
position corresponding  to  the  formula  Na2SO3.  This  salt 
is  sodium  sulphite,  and  its  relation  to  sulphurous  acid  is 
apparent  from  the  mode  of  its  formation : 

H2SO3  +  2  NaOH  ->  2  H2O  +  Na2SO3. 

372.  Sulphur   Trioxide   and   Sulphuric   Acid.    Although 
sulphur  dioxide  is  the  invariable  product  obtained  when  sul- 
phur burns,  it  is  not  the  only  oxide  of  sulphur.     Sulphur  tri- 
oxide,  SO3,  is  of  still  greater  importance  because  of  the  fact 
that  with  water  it  gives  sulphuric  acid  : 

SO3  +  H2O  ->  H2SO4. 

Sulphur  trioxide  is  obtained  by  allowing  sulphur  dioxide  to 
combine  with  more  oxygen  : 

2  SO2  +  O2  ->  2  SO3. 

There  is  a  good  deal  of  chemical  affinity  between  the  sulphur 
dioxide  and  the  oxygen  and  there  is  no  difficulty  in  keeping 
them  combined  (as  sulphur  trioxide)  if  once  they  can  be  in- 
duced to  unite.  But  the  reaction  by  which  they  combine 
is  sluggish,  and  very  little  sulphur  trioxide  can  be  obtained 
by  merely  bringing  the  two  gases  together,  even  if  they  are 
heated. 

373.  Manufacture   of   Sulphuric   Acid.     Since   sulphuric 
acid  is  one  of  the  most  extensively  used  of  chemicals,  its 


SULPHURIC  ACID  BY  CONTACT  PROCESS  351 

manufacture  is  of  the  greatest  importance.  There  are  two 
widely  used  processes  for  this.  In  both  processes,  the  first 
step  is  the  same,  namely,  burning  sulphur  to  sulphur  dioxide. 
Instead  of  free  sulphur,  iron  pyrite,  a  sulphide  of  iron  that  is 
high  in  sulphur  content,  is  often  burned. 

4  FeS2  +  11  O2  -»  8  SO2  +  2  Fe2O3. 

At  zinc  smelters,  zinc  sulphide  ore  must  be  roasted  to  convert 
it  to  zinc  oxide : 

2  ZnS  +  3  O2  ->  2  SO2  +  2  ZnO. 

Sulphuric  acid  plants  are  usually  set  up  in  such  places  so  as 
to  utilize  the  sulphur  dioxide. 

It  is  in  the  next  step  in  the  manufacture,  namely,  the  con- 
version of  sulphur  dioxide  to  sulphur  trioxide  that  the  chief 
difficulty  lies.  The  difficulty  is  overcome  by  the  use  of  a 
catalyzer,  which,  as  we  have  already  seen,  is  a  substance  which 
does  not  undergo  any  permanent  alteration  itself,  but  by  its 
mere  presence  causes  an  ordinarily  sluggish  reaction  to  be- 
come rapid.  The  difference  between  the  two  processes  lies 
mainly  in  the  different  catalytic  agents  used. 

374.  Contact  Process.  In  the  so-called  contact  process, 
the  catalytic  substance  is  platinum.  The  action  takes  place 
at  the  surface  of  the  metal ;  therefore,  to  present  as  large  a 
surface  as  possible  with  a  small  quantity  of  this  very  ex- 
pensive metal,  the  latter  is  spread  out  over  asbestos  fibers. 
The  asbestos  is  dipped  into  a  water  solution  of  platinum 
chloride,  it  is  dried  and  then  ignited  (heated  strongly).  The 
platinum  chloride  is  decomposed  and  the  metal  is  left  in  an 
extremely  fine  state  of  subdivision  on  the  surface  of  the 
fibers.  Tubes  are  now  packed  with  this  platinized  asbestos, 

B.  AND  W.  CHEM. 23 


352  SULPHUR 

and  when  these  tubes  are  kept  at  a  temperature  of  400°  C. 
and  a  mixture  of  sulphur  dioxide  and  air  is  passed  through 
them,  the  formation  of  sulphur  trioxide  takes  place  with  great 
readiness. 

To  obtain  sulphuric  acid,  it  is  only  necessary  to  mix  the 
sulphur  trioxide  with  water  (see  equation  on  page  350). 
Although  this  would  seem  to  be  a  very  simple  operation,  it 
does,  as  a  matter  of  fact,  offer  a  good  deal  of  difficulty,  for 
the  reaction  produces  great  heat ;  and  since  sulphur  trioxide 
is  very  volatile,  a  fearful  smoke  is  produced  if  the  mixing  is 
not  carried  out  with  the  greatest  caution.  In  practice,  the 
sulphur  trioxide  is  not  added  directly  to  water  but  rather  to 
a  fairly  concentrated  sulphuric  acid,  as  for  example  the  acid 
that  is  obtained  from  the  chamber  process. 

375.  Chamber  Process.  In  this  process,  oxides  of  nitro- 
gen are  used  as  the  catalytic  agent.  There  are  a  number  of 
different  oxides  of  nitrogen,  among  them  being  NO  and  NO2. 
Sulphur  dioxide  takes  oxygen  readily  from  the  higher  oxide, 
NO2,  but  the  lower  oxide  NO  thus  formed  takes  oxygen  at 
once  from  the  air  to  form  NO2  again.  The  NO2  again  gives 
oxygen  to  sulphur  dioxide  and  is  immediately  regenerated 
by  taking  on  more  oxygen  from  the  air,  and  thus  a  very  small 
amount  of  oxide  of  nitrogen  by  repeating  the  action  a  great 
many  times  can  "  carry  "  oxygen  to  a  large  amount  of  sul- 
phur dioxide,  whereas  sulphur  dioxide  by  itself  is  incapable 
of  combining  to  any  extent  with  the  oxygen. 

In  practice,  the  sulphur  dioxide  from  the  sulphur  or  pyrite 
burners,  together  with  an  excess  of  air,  is  conveyed  through 
a  tower  (see  diagram,  Fig.  71)  which  serves  partly  to  cool 
the  gases  and  partly  to  charge  them  with  oxides  of  nitrogen. 
They  are  then  allowed  to  pass  through  large  lead  chambers 
(sometimes  as  large  as  100  X  40  X  20  feet)  where  they  be- 


SULPHURIC   ACID   BY  CHAMBER  PROCESS 


353 


come  thoroughly  mixed  and  where  the  oxide  of  nitrogen  gets 
a  full  opportunity  to  carry  oxygen  to  the  sulphur  dioxide. 
To  aid  in  this  process  as  well  as  to  furnish  the  water  necessary 
to  convert  the  sulphur  trioxide  into  sulphuric  acid,  steam  or 
water  in  a  fine  spray  is  blown  into  the  lead  chambers.  The 
liquid  sulphuric  acid  collects  on  the  bottom  of  the  chamber 
and  from  there  is  siphoned  off. 

From  the  last  of  the  series  of  chambers,  there  is  little  left 
to  pass  out  as  gas  except  the  nitrogen  of  the  air  used  for  the 
oxidations,  and  the  oxides  of  nitrogen.  These  oxides  of 


— »*  Shows  Course  of  Su/fur  D/oxidanc/ Air 
«•— •        -  •  Oxides  ofWtrogrn 

•    Tower  Ac/d 


FIG.  71.  —  Chamber  Process  for  the  Manufacture  of  Sulphuric  Acid. 
(From  Thorp's  Outlines  of  Industrial  Chemistry.) 

nitrogen  are  poisonous  and  very  offensive,  and  it  would  not 
be  permitted  to  throw  them  out  into  the  atmosphere  even  if 
the  manufacturers  were  willing  to  waste  them.  The  es- 
caping gas,  therefore,  is  allowed  to  pass  up  through  a  tower 
filled  with  tiles  over  which  concentrated  sulphuric  acid  is 
trickling  down.  This  concentrated  acid  absorbs  the  oxides 
of  nitrogen  while  the  uncombined  nitrogen  passes  out  from 
the  top.  The  concentrated  acid  containing  the  oxides  of 
nitrogen  is  pumped  to  the  top  of  the  first-mentioned  tower, 
where  by  dilution  with  water  it  is  made  to  give  up  its  oxides 


354  SULPHUR 

of  nitrogen,  which  are  thus  reintroduced  into  the  process  to 
serve  again  as  oxygen  carriers. 

Under  perfectly  ideal  conditions,  a  limited  amount  of 
nitrogen  oxide  should  serve  forever  in  this  process.  Prac- 
tically, a  little  is  continually  being  lost ;  this  is  made  up  by 
introducing  into  the  gases  from  the  sulphur  burners  a  little 
vaporized  nitric  acid  which  decomposes  and  is  reduced  as 

folloWS  :  _  TTxrrA 

2  HNO3  ->  H2O  +  N2O5 
N2O5  +  3  SO2  ->  2  NO  +  3  SO3 

and  then  the  nitrogen  oxide  continues  to  act  as  oxygen 
carrier.  The  nitric  acid  used  is  generated  by  allowing 
sulphuric  acid  to  react  with  sodium  nitrate  (Chili  saltpeter) 
placed  in  a  small  pot  in  the  flue  between  the  burners  and  the 
first  tower. 

2  NaNO3  +  H2S04  ->  Na2SO4  +  2  HNO3. 

376.  Properties  of  Sulphur  Trioxide  and  Sulphuric  Acid. 

Sulphur  trioxide  is  unlike  sulphur  dioxide  in  that  it  is  not 
a  gas.  It  exists  in  two  modifications,  one  of  which  is  liquid 
at  ordinary  temperature,  the  other  of  which  is  a  fibrous 
white  crystalline  solid.  Both  of  these  forms  are  very  vola- 
tile and  if  left  open  to  the  atmosphere  evolve  dense  white 
fog  —  due  to  combining  with  the  water  vapor  in  the  air  to 
form  sulphuric  acid  which,  condensing  to  minute  droplets 
of  liquid,  gives  the  fog.  Sulphur  trioxide  has  a  great  af- 
finity for  water,  and  if  a  fragment  is  thrown  upon  water 
it  gives  a  violent  hissing  sound  like  the  quenching  of  red- 
hot  iron,  owing  to  the  heat  released  by  the  reaction. 

377.  Pure  sulphuric  acid,  H2SO4,  is  a  heavy,  oily  liquid 
of  specific  gravity  1.84.     Unlike  the  sulphur  trioxide,  from 
which  it  is  obtained,  it  is  not  at  all  volatile  at  ordinary  tern- 


SUMMARY  355 

perature.  It  mixes  with  water  in  all  proportions  and  much 
heat  is  developed  by  the  admixture.  Indeed,  sulphuric 
acid  has  a  good  deal  of  affinity  for  water.  For  example,  if 
air  or  any  other  gas  is  slowly  bubbled  through  concentrated 
sulphuric  acid,  it  is  practically  freed  from  water  vapor.  This 
is  a  convenient  method  for  drying  gases.  Again,  if  wood  or 
sugar  are  placed  in  contact  with  concentrated  sulphuric 
acid,  they  are  rapidly  charred  ;  the  hydrogen  and  oxygen 
of  the  wood  or  sugar  have  been  removed  in  the  form  of  water, 
while  the  carbon  is  left  and  gives  the  charred  appearance. 

When  it  is  diluted  with  a  considerable  amount  of  water, 
sulphuric  acid  is  a  very  strong  acid,  that  is  to  say,  it  is 
largely  dissociated  into  ions  : 


In  virtue  of  its  hydrogen  ions,  it  shows  to  a  marked  degree 
all  the  typical  properties  of  acids.  For  example,  all  metals 
standing  above  hydrogen  in  the  electromotive  series  react 
with  sulphuric  acid  solution  ;  hydrogen  is  set  free  and  a  sul- 
phate of  the  metal  is  left.  The  sulphates  of  most  of  the 
metals  are  soluble,  so  the  observed  effect  of  treating  a  metal 
with  sulphuric  acid  is  that  the  metal  dissolves  and  hydrogen 
gas  escapes  in  bubbles. 

Sulphuric  acid  neutralizes  bases  and  basic  oxides;  sul- 
phates are  thereby  formed,  but  there  is  no  effervescence,  be- 
cause the  hydroxyl  of  the  base,  or  the  oxygen  of  the  basic 
oxide,  is  present  to  combine  with  the  hydrogen  of  the  acid 
and  retain  it  in  the  form  of  water. 

SUMMARY 

Sulphur  is  a  typical  non-metal.     It  combines  directly  with  metals 

and  with  hydrogen  to  form  sulphides. 
Sulphur  combines  with  oxygen  to  form  oxides.     These  oxides  are 


356 


SULPHUR 


acid-forming,  which  fact  is  in  accord  with  the  non-metallic 

character  of  the  element  sulphur. 
Sulphur  dioxide  when  dissolved  in  water  gives  sulphurous  acid,  H2S03, 

a  weak  and  very  unstable  acid.     Neutralization  of  this  acid 

gives  a  sulphite  of  the  metallic  element  of  the  base. 
Sulphur  trioxide  when   dissolved  in  water  gives  sulphuric  acid, 

H2S04,  a  strong  and  stable  acid.     Neutralization  of  this  acid 

gives  a  sulphate. 

Much  information  concerning  the  relationships  among  the  most 
important  of  the  compounds  of  sulphur  is  shown  in  the  following 
table : 


SIMPLE 
COMPOUND 

VALENCE 

OP  SULPHUB 

ACID 

STRENGTH 
OP  ACID 

FORMULA  OF 
SODIUM  SALT 

H2S 

S 
S02 
S03 

-2 
0 

+  4 
+  6 

H2S 

Very  weak 

Na2S 

H2S03 

H2S04 

Weak 
Strong 

Na2SO3 
Na2S04 

Questions 

1.  In  which  of  the  two  great  subdivisions  of  the  elements  does 
sulphur  belong? 

2.  The  oxides  of  sulphur  yield  what  type  of  substance  on  union 
with  water  ? 

3.  Of  what  practical  use  is  sulphur? 

4.  What  is  meant  by  the  popular  expression  sulphur  water,  re- 
ferring to  spring  water  of  a  certain  type  ? 

5.  Which  of  the  three  elements  —  oxygen,  chlorine,  and  sulphur 
—  would  you  regard  as  the  weakest  non-metallic  element,  and  why  ? 

6.  Explain  briefly  how  the  difficulty  in  getting  S02  to   unite 
with  oxygen  is  overcome  in  (a)  the  contact  process,  (6)  the  chamber 
process  of  making  sulphuric  acid. 

7.  How  could  you  tell  by  mere  inspection  whether  the  acid  in 
a  bottle  was  concentrated  HC1  or  concentrated  H2S04? 

8.  In  order  to  appreciate  the  vast  extent  of  the  use  of  sulphuric 
acid,  try  to  find  in  the  reference  books  the  value  of  the  acid  manu- 


QUESTIONS  357 

factured  in  the  United  States  in  one  year.  See,  also,  if  you  can 
think  of  any  manufactured  article  which  has  not  had  sulphuric 
acid  used  either  directly  in  its  manufacture  or  indirectly  in  the  mak- 
ing of  the  things  used  in  making  the  article  in  question. 

9.  Compare  sulphur  with  oxygen  as  to  its  existing  in  two  or  more 
forms. 

10.  If  a  bottle  contained  sulphuric  acid,  what  effect  would  it  have 
upon  a  splinter  of  wood  clipped  into  it  ? 

11.  What  weight  of  sulphuric  acid  could  be  obtained  from  1000 
kilograms  of  iron  pyrite,  FeS2,  if  all  of  the  sulphur  of  the  pyrite  were 
utilized  ? 

12.  What  volume  would  the  sulphur  dioxide  (figured  under  stand- 
ard conditions)  obtained  as  the  direct  product  of  burning  the  1000 
kilograms  of  pyrite  occupy? 

13.  What  property  of  sulphuric  acid  makes  is  useful  for  drying 
gases  ? 

14.  Why  would  you  not  advise  trying  to  dry  ammonia  gas  by 
bubbling  it  through  sulphuric  acid  ? 


CHAPTER  XXX 
COMPOUNDS   OF  NITROGEN 

WE  have  already  learned  in  connection  with  our  study  of 
the  atmosphere  (Chapter  VII)  that  nitrogen,  although  it  com- 
prises four  fifths  of  the  air,  is  ordinarily  an  inert  element. 
Only  under  special  conditions  does  atmospheric  nitrogen 
enter  into  combination  with  other  elements.  When,  how- 
ever, it  is  once  in  combination,  it  enters  into  further  chemical 
change  with  great  readiness  and  it  is  largely  on  this  account 
that  combined  nitrogen  is  of  such  importance  in  the  chemistry 
of  animal  and  plant  life  as  well  as  of  explosives. 

COMPOUNDS  WITH  METALS  AND  WITH  HYDROGEN 

378.  Nitrides.     With  the  very  active  metals,  especially 
calcium  and  magnesium,  free  nitrogen  unites  with  consider- 
able ease  to  form  the  nitrides  CasN2  and  Mg3N2.     This  ac- 
tivity, however,  plays  no  part  in  the  economy  of  nature,  first, 
because  calcium  and  magnesium  are  too  active  to  be  found 
uncombined,  and  second,  because  the  ever  present  oxygen 
in  the  air  would  have  the  stronger  claim  upon  their  com- 
bining powers  if  the  metals  were  ever  free  to  act  with  the 
atmosphere. 

379.  Ammonia.     It  has  already  been  seen  that,  chemically, 
hydrogen  is  classed  with  the  metals.     The  nitride  of  hydro- 
gen, or  as  it  is  commonly  called,  ammonia,  NH3 ,  is  one  of  the 
very  important  compounds  of  nitrogen. 

358 


AMMONIA  359 

Ammonia  can  be  made  from  the  nitride  of  calcium  or  mag- 
nesium by  treating  it  with  water : 

Ca3N2  +  6  H20  ->  3  Ca(OH)2  +  2  NH3. 

Ammonia  can  also  be  made  to  a  slight  extent  by  the  direct 
combination  of  hydrogen  and  nitrogen  under  the  influence  of 
the  electric  spark  or  of  certain  catalyzers.  If  a  mixture 
of  three  volumes  of  hydrogen  and  one  volume  of  nitrogen  is 
subjected  to  a  spark  discharge,  about  2  per  cent  of  the  mix- 
ture combines.  That  the  tendency  to  combine  is  extremely 
small  is  further  shown  by  the  fact  that  if  ammonia  gas  is  sub- 
jected to  the  same  spark  discharge,  all  but  about  2  per  cent 
of  it  becomes  decomposed  into  the  free  elements. 

The  actual  source  of  all  the  ammonia  of  commerce  is  the 
so-called  gas  liquor  obtained  from  gas  works.  Coal  is  the 
fossil  remains  of  former  vegetation  and  still  contains  some  of 
the  combined  nitrogen  that  originally  existed  in  the  plants. 
When,  in  the  process  of  gas  manufacture,  the  coal  is  sub- 
jected to  destructive  distillation,  a  large  part  of  the  nitrogen 
passes  off  as  ammonia.  In  the  process  of  purification,  the 
gas  is  passed  through  water ;  and  since  ammonia  is  very  solu- 
ble in  water,  the  gas  liquor  retains  it  and  becomes  the  princi- 
pal source  of  this  compound.  The  ammonia  of  the  crude  gas 
liquor  is  expelled  by  heat  and  absorbed  in  sulphuric  acid, 
forming  ammonium  sulphate.  Crude  ammonium  sulphate 
is  much  used  as  a  fertilizer  on  account  of  its  nitrogen  content. 
It  is  also  used  as  a  starting  point  in  the  manufacture  of  pure 
ammonia  and  pure  ammonium  salts.  It  is  mixed  with  cal- 
cium hydroxide,  whereby  the  weaker  base  is  set  free : 

(NH4)2S04  +  Ca(OH)2  ->  CaSO4  +  2  NH4OH. 

On  heating  this  mixture,  ammonia  gas  passes  off,  and  after 
proper  purification,  it  is  absorbed  in  water  to  form  ammonia 


360  COMPOUNDS  OF  NITROGEN 

water,  or  after  drying  it  is  liquefied  by  compression  in  steel 
cylinders  for  use  in  refrigerating  plants,  or  it  is  absorbed  in 
hydrochloric  acid  to  form  ammonium  chloride  (sal  am- 
moniac). 

380.  Laboratory  Preparation  of  Ammonia.     For  prepar- 
ing ammonia  in  the  laboratory,  an  ammonium  salt,  usually 
ammonium  chloride,  is  mixed  with  a  strong  base,  calcium 
or  sodium  hydroxide,  and  the  mixture  is  heated.     Owing  to 
its  great  solubility,  the  gas  cannot  be  collected  over  water. 
It  can  be  caught  over  mercury,  or  for  simple  experiments  it 
may  be  run  into  dry  bottles  held  mouth  downward. 

381.  Properties  of   Ammonia.     Ammonia   is   a   colorless 
gas  of  an  extremely  penetrating  but  not  disagreeable  odor. 
It  is  dangerous  if  inhaled  in  quantity,  but  diluted  with  much 
air  it  is  harmless. 

Ammonia  is  very  soluble  in  water;  at  20° C.  and  atmos- 
pheric pressure,  one  volume  of  water  dissolves  710  volumes  of 
ammonia.  The  great  solubility  can  be  strikingly  shown  by 
the  same  fountain  experiment  (page  131)  that  was  given  to 
show  the  extreme  solubility  of  hydrogen  chloride.  The  solu- 
tion of  ammonia  is  mildly  alkaline,  due  to  the  formation  of 
ammonium  hydroxide,  which  is  a  comparatively  weak  base ; 

NH3  +  H2O  ->  NH4OH 

(See  Chapter  XIX.)  It  is  on  account  of  its  alkaline  char- 
acter that  ammonia  is  useful  for  cleaning  purposes. 

Dry  ammonia  gas  is  easily  liquefied.  At  20°  C.,  a  pressure 
of  8.4  atmospheres  is  sufficient  to  cause  its  condensation. 
At  atmospheric  pressure,  a  temperature  of  —  34°  C.  will  cause 
it  to  liquefy. 

382.  Refrigeration.     Liquid  ammonia  is  much  used  in  ice 
machines  (see  Fig.  72).     By  means  of  the  engine  and  pump, 
the  gas  is  compressed  and  liquefied.     The  heat  produced  by 


REFRIGERATION 


361 


the  compression  is  removed  by  passing  the  ammonia  through 
condenser  pipes  over  which  cold  water  is  flowing.  The 
liquid  ammonia  is  now  allowed  to  issue  through  a  small  ori- 
fice of  the  expansion  valve,  into  a  coil  of  larger  pipe.  Heat 

^  water  for  Condenser 


A    JL  A  JL    A  A  A  A  J.  J*  J  A  -A  A 


A  I   ,1 


pi 

Dl 


Expansion  Coil 


Expansion  Valve- 


Compressor.  Drine   Tank.- 

FIG.  72. — Ammonia  Refrigerating  Plant. 

is  necessary  to  cause  the  liquid  to  vaporize  just  as  it  is  neces- 
sary to  convert  water  into  steam.  This  liquid  ammonia  has 
already  been  deprived  of  much  of  the  heat  it  originally  pos- 
sessed as  a  gas.  In  order  to  vaporize,  then,  it  has  to  with- 
draw what  heat  it  can  get  from  its  surroundings  and  hence  the 


362  COMPOUNDS  OF   NITROGEN 

latter  are  cooled  to  a  very  low  degree.  The  expansion  pipes 
are  surrounded  by  a  non-freezing  brine  such  as  a  concentrated 
solution  of  calcium  chloride  (see  page  194) .  This  liquid  serves 
as  a  bath  in  which  to  immerse  vessels  containing  the  water 
that  is  to  be  converted  to  ice.  Or  the  liquid  itself  may  be 
circulated  through  pipes  in  the  refrigerating  plant.  As  fast 
as  the  ammonia  vaporizes  in  the  expansion  pipes,  it  is  with- 
drawn by  means  of  the  pump  and  is  forced  again  through  the 
same  cycle  of  operations. 

383.  Ammonium   Salts.      Like   other   bases,   ammonium 
hydroxide  is  capable  of  neutralizing  acids,  as,  for  example, 

NH4OH  +  HC1  -+  NH4C1  +  H2O, 

and  thus  we  have  a  series  of  ammonium  salts  including  the 
chloride,  the  sulphate,  the  nitrate,  the  carbonate,  etc. 

Ammonium  chloride,  NH4C1,  is  commonly  known  as  sal 
ammoniac,  and  its  solution  is  much  used  in  bell-ringing 
electric  batteries. 

Ammonium  carbonate  is  used  in  smelling  salts.  Lumps 
of  the  solid  salt  are  covered  with  alcohol  and  a  little  aromatic 
oil  such  as  oil  of  lavender  is  added.  The  salt  is  unstable  and 
is  continually  decomposing  into  ammonia  and  carbon  dioxide. 
Thus  the  smelling  salts  possess  a  powerful  odor  of  ammonia. 

Ammonium  nitrate  is  used  in  considerable  quantities  in 
explosives.  It  leaves  no  solid  products,  as  does  the  potas- 
sium nitrate  used  in  gunpowder ;  it  is  serviceable  as  one  of 
the  ingredients  of  some  kinds  of  blasting  powder. 

COMPOUNDS  WITH  OXYGEN 

384.  A  number  of  compounds  of  nitrogen  and  oxygen  are 
known,  nevertheless  it  is  a  matter  of  great  difficulty  to  make 
the  two  elements  combine  directly.     Ordinarily,  they  remain 


OXIDES   OF   NITROGEN 


363 


mixed  together  in  the  air  without  the  least  trace  of  interac- 
tion, but  at  the  high  temperature  that  accompanies  the  pas- 
sage of  an  electric  spark  they  combine  to  a  slight  extent. 
Thus  a  small  amount  of  nitrogen  oxide  is  formed  during 
thunderstorms  in  the  paths  of  the  lightning  flashes.  With 
the  rain  water  it  is  converted  to  nitric  acid  and  contributes 
thus  a  small  share  toward  fertilizing  the  soil. 

An  artificial  method  of  making  nitric  acid  from  the  air  l>v 
means  of  electric  sparks  has  already  been  referred  to  in  sc<> 
tion  75.  The  success  of  this  process  depends  largely  on  being 
able  to  withdraw  the  nitric  oxide  quickly  from  the  influence  of 
the  spark  discharge  before  it  can  again  be  decomposed  into 
its  elements. 

385.  There  are  five  distinct  oxides  of  nitrogen;  their 
composition  and  some  of  their  properties  are  shown  in  the 
following  table : 


NAME 

FORMULA 

PROPERTIES 

Nitrous  Oxide 

N20 

Colorless  gas 

Nitric  Oxide 

NO 

Colorless   gas  ;    unites  spon- 

taneously with  oxygen  to 

form  the  red  gas  N02. 

Nitrogen  Trioxide 

N203 

Very  unstable,  anhydride  of 

nitrous  acid,  HNO2. 

Nitrogen  Tetroxide 

N02  (or  N204) 

Brownish  red  gas  ;    dissolved 

in  water  gives  a  mixture  of 

nitrous  and  nitric  acids. 

Nitrogen  Pentoxide 

N205 

White  unstable  solid,  anhy- 

dride of  nitric  acid. 

386.  Nitrous  Oxide,  N2O,  may  be  prepared  by  heating 
ammonium  nitrate  which  decomposes  according  to  the 
reaction  NH4NO3  -*  N2O  +  2  HA 


364  COMPOUNDS  OF   NITROGEN 

This  gas  is  much  used  by  dentists  and  surgeons  to  produce 
anaesthesia  for  short  operations.  Nitrous  oxide  is  a  colorless 
gas,  it  is  somewhat  soluble  in  water,  and  it  has  a  sweetish 
taste.  It  causes  a  glowing  splinter  of  wood  to  burst  into 
flame  and  it  supports  the  combustion  of  sulphur,  phos- 
phorus, and  other  combustibles  with  much  the  same  vigor 
as  does  pure  oxygen ;  but  the  animal  body  is  unable  to  obtain 
oxygen  from  it  and  so  it  does  not  sustain  life.  It  is  usual  to 
mix  it  with  air  when  producing  anaesthesia  so  as  not  entirely 
to  deprive  the  patient  of  oxygen. 

387.    Nitric  Oxide,  NO.     For  laboratory  study,  this  gas 

is  made  by  the  action  of  copper  on  nitric  acid.     The  nitric 

acid  is  reduced,  that  is,  it  has  oxygen  taken  away  from  it 

by  the  copper :      2  HNQ3  ->  H2O  +  2  NO  +  3  O  (1) 

3  O  +  3  Cu  -+  3  CuO 

The  copper  is  thereby  oxidized  to  copper  oxide,  a  basic  oxide. 
We  do  not  see  any  appearance  of  this  oxide,  however,  because 
as  fast  as  it  is  formed  it  reacts  with  the  surplus  nitric  acid  to 
form  the  salt,  copper  nitrate  : 

3  CuO  +  6  HNO3  ->  3  Cu(NO2)2  +  3  H2O.  (2) 

Reactions  (1)  and  (2)  occur  simultaneously  and  are  dependent 
on  each  other ;  if  their  equations  are  added,  the  equation  for 
the  entire  change  becomes  : 

3  Cu  +  8  HNO3  ->  3  Cu(NO3)2  +  4  H2O  +  2  NO. 

Nitric  oxide  is  a  colorless  gas  and  is  but  slightly  soluble 
in  water,  with  which  it  does  not  react.  It  does  not  support 
ordinary  combustion,  but  phosphorus  which  has  been  pre- 
viously kindled  will  continue  to  burn  in  nitric  oxide. 

Its  most  remarkable  property  is  that  it  combines  spon- 
taneously with  oxygen  in  the  cold,  thereby  producing  nitro- 


NITROGEN  TETROXIDE  365 

gen  tetroxide,  a  deep  brownish  red  gas  of  a  suffocating  odor. 
When,  for  example,  colorless  nitric  oxide  emerges  from  a 
delivery  tube  into  the  open  air,  it  changes  as  if  by  magic 
into  the  deep  red  vapor  of  nitrogen  tetroxide. 

388.  Nitrogen  Tetroxide,  NO2,  is,  as  stated,  a  deep  red  gas 
at  ordinary  temperature.  Its  odor  is  suffocating  and  not 
altogether  dissimilar  to  that  of  chlorine.  On  heating,  its 
color  becomes  deeper  red  ;  on  cooling,  its  color  becomes  paler 
until  the  gas  condenses  to  a  pale  yellow  liquid.  This  liquid 
boils  at  22°  C.  and  gives  the  characteristic  reddish  fumes. 
At  —12°  C.  it  freezes  to  an  almost  colorless  solid. 

The  changes  of  color  are  believed  to  be  due  to  a  change  in 
the  size  of  the  molecule.  The  red  gas  is  believed  to  consist 
of  molecules  of  NO2.  At  144°C.,  the  vapor  density  corre- 
sponds to  the  molecular  weight  46  of  NO2.  At  lower  tem- 
peratures the  density  is  higher,  indicating  a  mixture  of  NO2 
and  N2C>4  molecules.  The  liquid  is  supposed  to  consist 
entirely  of  N2O4. 

Nitrogen  tetroxide  is  a  powerful  oxidizing  agent,  support- 
ing the  combustion  of  most  of  the  ordinarily  combustible 
substances.  It  dissolves  in  water  and  at  the  same  time  reacts 
with  it  to  form  a  mixture  of  nitrous  and  nitric  acids : 

2  N02  +  H20  ->•  HN02  +  HNO3. 

If  the  water  is  warm,  the  unstable  nitrous  acid  decomposes 
as  follows : 

3  HNO2  ->  H2O  +  2  NO  +  HNO3. 

The  escaping  NO  can  form  further  NO2  when  it  comes  in 
contact  with  the  air.  These  reactions  play  an  important 
part  in  the  process  of  making  nitric  acid  from  the  air,  for  the 
nitric  oxide  first  formed  in  the  path  of  the  electric  spark  re- 


366  COMPOUNDS  OF  NITROGEN 

acts  with  oxygen  to  form  nitrogen  tetroxide  and  the  latter 
reacts  with  water. 

389.  Nitric  Acid,  HNO3,  is  the  most  important  compound 
of  nitrogen.     The  great  bulk  of  it  is  made  from  crude  sodium 
nitrate,  NaNOg,  which  is  found  in   extensive   deposits    in 
Chili,  South  America,  and  is  hence  known  as  Chili  saltpeter. 
The  origin  of  the  deposit  will  be  spoken  of  in  a  later  part  of 
this  chapter.     Although  this  deposit  is  of  great  magnitude, 
it  is,  nevertheless,  being  used  so  extensively  that  it  cannot 
last  for  a  great  many  years  longer,  and  eventually  the  world 
must  look  to  other  sources  for  its  nitrate  supply,  very  prob- 
ably to  the  electrical  method  of  "  fixing  "  the  atmospheric 
nitrogen. 

In  the  manufacture  of  nitric  acid,  Chili  saltpeter  is  treated 
with  concentrated  sulphuric  acid  and  the  mixture  is  heated. 
Nitric  acid,  being  volatile,  distils  off,  while  sodium  acid  sul- 
phate is  left  in  the  retort : 

NaN03  +  H2SO4->  NaHSO4  +  HNO3. 

The  vapor  of  the  nitric  acid  is  passed  through  air-cooled 
earthenware  or  glass  vessels  in  which  it  condenses  to  a  liquid. 

390.  Properties  of  Nitric  Acid.     Nitric  acid,  when  pure, 
is  a  colorless  liquid  of  specific  gravity  1.5,  but  it  readily  de- 
composes to  some  extent,  especially  if  warmed  or  exposed  to 
the  light.     Some  nitrogen  tetroxide  is  formed,  and  this  colors 
the  liquid  yellow  or  even  reddish,  and  red  fumes  of  it  are  fre- 
quently observed  above  the  liquid  in  the  bottle.     Commercial 
nitric  acid  has  a  specific  gravity  of  1 .42  and  contains  only  60 
to  70  per  cent  of  HNO3,  the  rest  being  water. 

One  of  the  most  marked  properties  of  nitric  acid  is  its  effect 
on  organic  matter.  It  attacks  the  skin,  turning  it  bright 
yellow,  and  when  concentrated,  it  makes  severe  and  deep 


PROPERTIES  OF  NITRIC   ACID  367 

burns.  Fabrics  are  at  once  destroyed  if  concentrated  nitric 
acid  is  spilled  on  them.  An  ignited  piece  of  charcoal  will 
continue  to  burn  when  thrust  below  the  surface  of  concen- 
trated nitric  acid. 

Nitric  acid  is  a  strong  acid  in  the  same  sense  that  hydro- 
chloric and  sulphuric  acids  are  strong,  that  is,  it  is  very 
highly  ionized  in  its  dilute  solution  and  thus  furnishes  an 
abundance  of  hydrogen  ions.  It  has,  however,  other  im- 
portant properties  besides  those  caused  by  the  hydrogen  ions ; 
for  example,  nitric  acid  readily  attacks  copper,  silver,  and  mer- 
cury, metals  which  stand  below  hydrogen  in  the  electromotive 
series  and  which  are  unacted  upon  by  hydrochloric  acid 
or  dilute  sulphuric  acid.  No  hydrogen,  however,  is  liberated 
by  the  action  of  nitric  acid  on  these  metals ;  a  gas  is  evolved 
which  is  colorless  in  the  bubbles  which  rise  through  the  liquid 
but  turns  red  when  the  bubbles  burst  and  the  gas  comes  in 
contact  with  the  air.  This  gas  obviously  is  nitric  oxide. 
The  reaction  of  copper  with  nitric  acid  has  already  been 
explained  in  section  387.  Nitric  acid,  coming  as  it  does  from 
the  anhydride  N2O5,  contains  an  abundant  supply  of  oxygen 
and  is  able  to  release  a  large  part  of  it  very  readily.  Thus 
it  oxidizes  metals.  It  is  most  often  reduced  thereby  to 
nitric  oxide:  NIQ, _*  2  NO  +  3  O. 

After  oxidizing  the  metal,  nitric  acid  begins  to  display  its 
acid  function  in  that  it  neutralizes  the  basic  oxide,  forming 
water  and  a  salt  (nitrate)  of  the  metal.  In  just  the  same 
way,  hydrochloric  and  dilute  sulphuric  acids  react  with  the 
oxides  of  the  metals  copper,  silver,  and  mercury  and  form  the 
corresponding  chlorides  and  sulphates,  although  the  latter 
acids  have  no  action  at  all  on  the  uncombined  metals. 

When  metals  standing  above  hydrogen  in  the  electro- 

B.  AND  W.  CHEM. 24 


368  COMPOUNDS  OF  NITROGEN 

motive  series  react  with  nitric  acid,  there  is  no  reason  why 
hydrogen  should  not  be  displaced.  Usually,  however,  it 
does  not  escape  as  such,  for  nitric  acid,  being  an  oxidizing 
agent,  catches  the  hydrogen  in  the  moment  of  its  formation, 
when  it  is  particularly  active,  and  oxidizes  it  to  water.  A 
very  active  metal  like  magnesium  can  sometimes,  when  the 
acid  is  dilute,  produce  hydrogen  so  copiously  that  some  of 
it  escapes  the  oxidizing  action  of  the  nitric  acid  and  is  evolved 
as  hydrogen  gas. 

Hydrogen  is  able  to  react  with  nitric  acid  only  when  it  is 
in  the  particularly  active  condition  at  the  moment  of  its  for- 
mation at  the  surface  of  the  metal.  A  stream  of  hydrogen 
formed  in  a  separate  generator  may  be  passed  freely  into 
nitric  acid  without  the  least  reaction  taking  place.  Thus 
whatever  hydrogen  can  get  fairly  formed  and  escape  from  the 
surface  of  the  magnesium  before  it  is  oxidized  will  be  evolved 
with  the  other  reaction  products. 

It  has  already  been  seen  under  the  preparation  of  sulphur 
dioxide  that  hot  concentrated  sulphuric  acid  can  react  as  an 
oxidizing  agent  upon  copper  and  that  the  reduction  product 
of  the  sulphuric  acid  is  sulphur  dioxide  in  the  same  way  that 
the  reduction  product  of  nitric  acid  is  nitric  oxide.  Sulphur 
is  a  more  active  element  than  nitrogen ;  in  consequence,  its 
oxides  are  more  stable,  and  the  oxides,  or  the  acids  derived 
from  them,  are  more  difficult  to  reduce.  Cold  or  dilute  sul- 
phuric acid  thus  is  without  action  on  copper,  whereas  nitric 
acid  reacts  when  it  is  both  cold  and  dilute. 

391.  Reduction  of  Nitric  Acid  to  Ammonia.  The  usual 
reduction  product  of  nitric  acid  is  nitric  oxide,  NO.  If  we 
should  imagine  the  reduction  to  be  carried  further,  we  might 
expect  to  obtain  free  nitrogen.  If,  however,  the  reduction 
gets  as  far  as  this,  it  seldom  stops  here,  but  goes  a  step  further. 


REDUCTION  OF  NITRIC   ACID   TO   AMMONIA        369 

It  may  be  wondered  how  this  is  possible,  how  anything  not 
possessed  can  be  taken  away  !  Electrically  speaking,  we  have 
already  seen  that  addition  of  positive  is  equivalent  to  the 
taking  away  of  n  gative,  and  vice  versa.  The  same  is  true  chem- 
ically, and  the  addition  of  positive  hydrogen  is  considered 
reduction  as  much  as  the  taking  away  of  negative  oxygen. 

Now  when  nitric  acid  is  treated  with  magnesium  or  zinc 
or  any  metal  standing  high  in  the  electromotive  series,  there 
is  aways  the  possibility  of  the  setting  free  of  hydrogen. 
This  hydrogen  is  especially  active  at  the  moment  of  its  for- 
mation ;  and  if  it  finds  free  nitrogen  at  the  moment  of  its  lib- 
eration by  the  reduction  of  nitric  oxide,  it  combines  with  it 
to  form  ammonia. 

Let  us  consider  the  actual  experiment  of  treating  zinc 
turnings  with  cold  dilute  nitric  acid.  The  reaction  is  not  so 
violent  as  with  more  concentrated  acid,  and  little  or  no  red 
fumes  of  nitrogen  dioxide  can  be  seen  escaping.  We  suspect 
ammonia  is  formed,  but  we  cannot  see  it  or  perceive  its  odor. 
In  fact,  if  we  reflect  a  moment  we  should  not  expect  to  find 
free  ammonia,  for  ammonia  is  basic ;  it  forms  ammonium  hy- 
droxide with  some  of  the  water  present  and  that  in  turn  reacts 
with  the  surplus  nitric  acid  to  form  ammonium  nitrate. 
With  this  idea  in  mind,  we  can  start  to  show  the  existence  of 
the  ammonia  in  combination.  It  is  only  necessary  to  add  to 
the  solution  a  stronger  base  which  will  liberate  the  weaker 
base  from  its  salt : 

Na+  OH"  +  NH4+  NO3-  ->  Na+  NOT  \  +  (NH4OH). 
Then,  especially  on  warming  the  solution,  ammonium  hy- 
droxide breaks  up  into  ammonia  and  water : 

NH4OH  -^  NH3  f  +  H20, 
and  the  odor  of  the  escaping  ammonia  gas  is  evident. 


370  COMPOUNDS  OF  NITROGEN 

The  reduction  of  nitric  acid  by  copper  was  compared  with 
the  similar  reduction  of  sulphuric  acid.  It  is  also  true  of 
sulphuric  acid  that  it  can  be  reduced  to  a  stage  further  than 
the  lower  oxide  or  even  than  free  sulphur.  When  fairly  con- 
centrated, warm  sulphuric  acid  is  treated  with  zinc  or  mag- 
nesium, a  considerable  amount  of  hydrogen  sulphide  is  ob- 
tained among  the  other  gases  that  escape.  The  active 
hydrogen  liberated  by  the  metal  adds  itself  to  the  sulphur. 

392.  Explosive  Compounds  of  Nitrogen.     It  has  already 
been   stated   that   nitric  acid  is  a  strong  oxidizing  agent 
because  it  is  unstable  and  gives  up  its  oxygen  readily.     It  is 
also  true  of  nitrogen  compounds  in  general  that  they  are  un- 
stable.    Many  of  the  compounds  of  nitrogen  are  so  unstable 
that  they  are  explosive,  that  is  to  say,  when  they  start  to  de- 
compose they  do  so  with  violence. 

393.  Gunpowder  is  a  mixture  of  potassium  nitrate  with 
powdered   charcoal   and   sulphur.     Potassium   nitrate,   like 
nitric  acid,  gives  up  oxygen  readily,  and  with  carbon  present 
to  unite  with  the  oxygen,  the  reaction,  if  once  started,  be- 
comes violent  within  an  instant.     Nitrogen  gas  and  carbon 
dioxide,  with  possibly  some  carbon  monoxide,  are  gaseous  prod- 
ucts of  the  reaction ;  and  the  sudden  production  of  a  large 
quantity  of  these  gases  added  to  the  expansive  effect  of  the 
great  heat  of  the  reaction  accounts  for  the  power  of  the  ex- 
plosion. 

394.  Cellulose  Nitrate ;  Guncotton.     Most  of  the  modern 
high  explosives  are  made  by  the  carefully  regulated  action  of 
nitric  acid  on  some  of  the  organic  compounds  of  carbon. 
Guncotton  is  one  of  these  and  it  is  made  by  the  action  of  a 
mixture  of  concentrated  nitric  and  sulphuric  acids  on  pure 
cotton  fiber,  which  is  almost  pure  cellulose  (CeHioOs)^ 

2  CeHlo05  +  6  HN03->  C12H1404(N03)6  +  6  H2O. 


NITROGLYCERIN  371 

As  seen  in  the  equation,  water  is  one  of  the  products  of  the 
reaction;  the  function  of  the  sulphuric  acid  is  to  appro- 
priate this  water,  which  would  otherwise  accumulate  and 
check  the  reaction. 

When  guncotton  explodes,  a  rearrangement  of  the  atoms 
takes  place  so  that  the  simpler  and  much  stabler  molecules 
CO,  CO2,  H2O,  and  N2  are  formed.  All  of  the  products  are 
gases  and  hence  no  smoke  is  produced.  Guncotton  is  the 
basis  of  modern  smokeless  powder. 

Guncotton  is  fairly  safe  to  handle,  for  it  is  stable  enough 
to  withstand  considerable  heat  and  shock  without  exploding. 
It  is  customarily  fired  by  means  of  a  detonating  cap  of  mer- 
curic fulminate  (Hg(ONC)2  —  also  a  nitrogen  compound). 
The  shock  of  the  explosion  of  the  cap  detonates,  or  explodes, 
the  guncotton.  If  not  detonated  but  simply  ignited  with  a 
match  in  the  open,  guncotton  burns  rapidly  but  usually 
without  trace  of  explosion. 

Lower  nitrates  of  cellulose  are  made  by  using  less  con- 
centrated nitric  acid ;  they  are  far  less  explosive  than  the 
hexanitrate  and  find  use  in  making  collodion,  celluloid,  and 
artificial  silk.  Articles  made  from  them  are  extremely  in- 
flammable. 

395.  Nitroglycerin,    C3H5(NO3)3,    is    made    by    treating 
glycerin  (glycerol)  in  the  same  way  as  cellulose  is  treated  in 
making  guncotton.     Nitroglycerin  is  far  more  dangerous  to 
handle  because  it  explodes  with  very  slight  provocation. 
Dynamite  is  made  by  impregnating  diatomaceous  earth  or 
other  porous  material  with  nitroglycerin.     It  is  far  safer  to 
handle  than  the  clear  nitroglycerin,  and  it  gives  nearly  as 
powerful  an  explosion. 

396.  Fixation  of  Nitrogen  by  Aid  of  Bacteria.     It  was 
stated  in  Chapter  VII  that  much  atmospheric  nitrogen  is 


372  COMPOUNDS  OF  NITROGEN 

continually  finding  its  way  into  the  combined  form  through 
the  agency  of  the  life  processes  of  certain  bacteria  which  live 
on  the  roots  of  plants  belonging  to  the  legume  family  (peas, 
beans,  lentils,  alfalfa,  and  clover  belong  to  this  family). 
The  plants  assimilate  the  nitrogen  thus  combined  and  con- 
vert it  into  the  complex  compounds  with  carbon,  hydrogen, 
and  oxygen  which  go  to  make  up  the  protoplasm  of  the  plant 
cells,  the  chlorophyll  or  green  coloring  matter,  etc.  The 
seeds  usually  are  especially  rich  in  nitrogenous  substances. 
Peas  and  beans  thus  make  valuable  food  material  for  men  and 
animals. 

The  total  amount  of  nitrogen  combined  by  all  of  the  arti- 
ficial means  as  well  as  by  the  lightning  discharges  in  thunder 
storms  is  entirely  insignificant  in  comparison  with  that  fixed 
by  bacterial  action. 

By  plowing  under  a  crop  of  clover,  or  other  legume,  nitrog- 
enous matter  can  be  introduced  into  the  soil,  where  it  be- 
comes finally  converted  into  nitrates.  The  latter  change  is 
accomplished  through  the  oxidizing  agency  of  the  air  in  the 
presence  of  the  alkaline  material  of  the  soil  and  with  the  aid 
of  the  various  bacteria  of  decay.  Nitrogen  in  the  form  of 
nitrates  is  available  for  the  nutrition  of  other  kinds  of  plants, 
such  as  wheat  and  oats,  which  cannot  take  nitrogen  from  the 
air.  Unless  some  such  method  of  getting  nitrogenous  matter 
into  the  soil  is  adopted,  grain  and  other  crops  will  not  thrive 
after  a  few  years  without  artificial  fertilizers  containing  nitro- 
gen, such  as  animal  manure  or  Chili  saltpeter  (sodium 
nitrate) . 

Farmers  generally  understand  the  necessity  of  keeping 
up  the  nitrogen  content  of  the  soil,  and  they  frequently  plant 
clover,  alfalfa,  or  other  legumes,  since  this  method  is  cheaper 
than  buying  expensive  sodium  nitrate. 


THE   NITROGEN   CYCLE   IN  NATURE  373 

397.  Origin  of  Saltpeter  Beds.     The  origin  of  the  Chili 
saltpeter  deposits  is  not  known,  but  it  is  generally  agreed  that 
the  nitrogen  is  of  organic  origin,  either  animal  or  vegetable. 
When  organic  material  decays,  as  in  the  damp  soil  of  cattle 
yards,  some  of  the  nitrogen  is  oxidized  to  nitric  acid,  which 
gives  nitrates  with  the  alkaline  material  in  the  soil ;    if  the 
surface  of  the  soil  is  dry,  a  whitish  crust  made  up  of  small 
crystals  of  potassium  nitrate   (saltpeter)   often  forms.     In 
some  localities,  notably  in  India,  the  soil  near  the  villages 
where,  due  to  imperfect  sewage  disposal,  much  nitrogenous 
matter  decomposes,  is  extracted  with  water  every  few  years 
and  the  nitrates,  chiefly  calcium  and  potassium  nitrates,  are 
obtained  by  evaporation. 

The  Chili  saltpeter  without  doubt  arose  through  the 
oxidation  of  vast  deposits  of  nitrogenous  organic  matter. 
The  soil  in  that  region  must  have  abounded  in  soda  rather 
than  potash  or  lime.  The  peculiar  formation  of  the  plateau 
and  the  absence  of  rain  accounts  for  the  retention  of  the 
deposit. 

398.  The  Nitrogen  Cycle  in  Nature.     When  nitrogenous 
compounds  of  either  plant  or  animal  origin  decay  under  usual 
conditions,  the  carbon  is  oxidized  to  carbon  dioxide  and  the 
hydrogen  to  water ;  the  nitrogen  appears  at  first  in  the  form 
of  ammonia  but  this  is  oxidized  partly  to  free  nitrogen  which 
escapes  as  gas  and  partly  to  nitrate.     We  have  thus  a  com- 
plete cycle  through  which  nitrogen  passes  from  the  free  state 
in  the  air,  through  the  combined  state  in  the  legume,  from 
the  legume  to  other  states  of  combination  in  plant  and  animal 
matter,  and  finally  through  decay  back  again  to  the  atmos- 
phere.    If  the  legume  is  eaten  by  animals  the  nitrogen  is 
changed  into  the  various  nitrogenous  substances  of  the  animal 
body ;  the  decay  of  either  the  legumes  or  of  the  animal  matter 


374 


COMPOUNDS  OF   NITROGEN 


yields  in  part  free  nitrogen  and  in  part  nitrates,  which  in  the 
soil  serve  to  nourish  other  vegetation,  which  in  turn  nourishes 

animals,  or  else  de- 
cays and  forms  ni- 
trates again  or  yields 
back  free  nitrogen  to 
the  air. 

In  this  respect,  ni- 
trogen resembles  car- 
bon,  which  was 
shown  in  Chapter  VI 
to  go  through  a  cycle 
of  changes  in  which 
plants  and  animals 
and  the  carbon  di- 
oxide of  the  air  play 
a  part. 

The  amount  of 


FIG.  73.  —  Nitrogen  Cycle. 


nitrogen  involved  in  these  changes  is  smaller  than  the  amount 
of  carbon  involved  in  the  carbon  cycle.  Nevertheless,  the 
same  sort  of  equilibrium  is  maintained.  The  relative 
amounts  of  combined  nitrogen  and  of  uncombined  atmos- 
pheric nitrogen  remain  constant  because  the  amount  enter- 
ing combination  is  just  equalled  by  the  amount  which  is  all 
the  time  being  returned  to  the  atmosphere. 

SUMMARY 

Free  nitrogen  is  very  inactive,  but  in  combination,  nitrogen  enters 
freely  into  reactions  and  its  compounds  are  of  great  impor- 
tance. 

Nitrides  and  Ammonia.  Nitrogen  combines  with  the  very  active 
metals  to  form  nitrides.  The  nitride  of  hydrogen  is  known  as 
ammonia.  It  unites  with  water  to  form  ammonium  hydroxide, 


QUESTIONS  375 

a  rather  weak  base.  Nitrides  other  than  ammonia  are  of  little 
practical  importance.  Ammonia  is  obtained  as  a  by-product 
from  gas  works. 

Oxides.  Oxygen  forms  five  distinct  compounds  with  nitrogen. 
Nitrogen  pentoxide,  N20s,  is  the  anhydride  of  nitric  acid. 

Nitric  acid  is  the  most  important  derivative  of  the  oxygen  com- 
pounds. It  is  a  strong  acid  and  also  a  strong  oxidizing  agent. 
When  it  oxidizes  other  substances,  it  is,  according  to  condi- 
tions, itself  reduced  to  N02,  NO,  N2,  or  even  in  extreme  cases 
to  NH3.  Nitric  acid  is  obtained  principally  by  treating  Chili 
saltpeter,  NaN03,  with  sulphuric  acid. 

Explosives.  Oxygen  compounds  of  nitrogen  and  their  derivatives 
are  very  unstable  and  form  the  basis  of  all  the  common  ex- 
plosives. 

A  natural  cycle  of  changes  is  undergone  by  nitrogen  in  much  the 
same  manner  as  by  carbon.  The  decay  of  nitrogenous  mate- 
rial returns  to  the  atmosphere  an  amount  of  nitrogen  just 
equalled  by  that  entering  combination  chiefly  under  the  in- 
fluence of  leguminous  plants. 

Questions 

1.  When  powdered  magnesium  is  heated  in  a   crucible  with 
loosely  fitting  lid  and  water  is  added  to  the  residue. after  cooling, 
some  ammonia  is  set  free,  as  can  be  told  by  the  odor.     Account  for 
the  formation  of  the  ammonia. 

2.  Explain  the  use  of  anhydrous  ammonia  in  making  artificial  ice. 

3.  A  flask  is  filled  with  a  colorless  gas,  but  when  the  stopper  is 
taken  out  the  gas  in  the  neck  of  the  flask  turns  brown.     What  is  the 
gas? 

4.  How  does  the  action  of  nitric  acid  on  copper  differ  from  the 
action  of  hydrochloric  acid  on  zinc? 

5.  Write  the  reaction  of  nitric  acid  with  silver,  remembering  that 
the  formula  of  silver  oxide  is  Ag20. 

6.  Show  by  equations  how  ammonium  nitrate  can  be  one  of  the 
products  of  the  action  of  nitric  acid  on  zinc.     What  gaseous  reduc- 
tion products  may  escape  at  the  same  time  that  the  ammonium, 
nitrate  is  being  formed  ? 


376  COMPOUNDS  OF  NITROGEN 

7.  Why  do  farmers  sometimes  plant  clover  only  to  plow  it  under 
without  harvesting  the  crop  ? 

8.  What  weight  of  calcium  hydroxide  is  necessary  to  react  with 
100  grams  of  ammonium  sulphate  in  the  preparation  of  ammonia  ? 

9.  What  volume  would  the  ammonia  gas  occupy  under  standard 
conditions  ? 

10.  What  weight  of  28  per  cent  ammonia  solution  would  be  ob- 
tained ? 

11.  What  volume  would  this  solution  occupy  if  its  specific  gravity 
is  0.90? 

12.  What  weight  of  70  per  cent  HNOs  should  be  taken  to  just 
dissolve  190.8  grams  of  copper  ? 

3  Cu  +  8  HN03  -*  3  Cu(N03)2  +  2  NO  +  4  H20. 

13.  At  standard  conditions,  what  volume  of  NO  gas  would  be 
evolved  in  question  12? 

14.  With  what  volume  of  air  (20  per  cent  oxygen)  should  this 
nitric  oxide  be  mixed  in  order  to  change  it  all  to  N02? 

15.  Formulate  as  well  as  you  are  able  the  reactions  for  the  ex- 
plosion of  ordinary  gunpowder  and  of  smokeless  powder.      Explain 
why  the  former  produces  smoke  and  the  latter  does  not. 

16.  Heat  is  evolved  when  N20  decomposes  into  nitrogen  and  oxy- 
gen.   Consider  this  fact  and  explain  why  N20  should  support  com- 
bustion practically  as  well  as  pure  oxygen. 


CHAPTER  XXXI 

THE   HALOGEN   FAMILY;     THE   PERIODIC   SYSTEM 

399.  Natural  Families  of  the  Elements.  With  the  eighty 
or  more  elements  which  are  known  to  exist,  it  would  be  strange 
if  there  were  not  resemblances  between  some  and  dissimilari- 
ties between  others.  A  comparison  of  all  the  elements  has 
shown  that  what  would  thus  be  expected  is  amply  fulfilled 
by  the  facts,  for  the  elements  seem  to  arrange  themselves 
naturally  into  families,  and  the  elements  within  a  family 
show  to  a  marked  degree  certain  traits  which  are  character- 
istic of  that  family.  One  of  these  families,  the  so-called 
halogen  family,  forms  the  subject  matter  for  this  chapter. 

The  best-known  element  of  this  family  is  chlorine.  The 
other  elements  are  fluorine,  bromine,  and  iodine.  All  four 
elements  form  salts  which  are  extremely  alike  in  character. 
Thus  the  sodium  salts  of  fluorine,  bromine,  and  iodine  are 
very  similar  to  common  salt,  which  is  the  sodium  salt  of 
chlorine,  and  hence  the  name  halogen,  which  comes  from 
the  Greek  and  means  salt-former. 

It  is  recalled  that  chlorine  is  a  gas  with  "a  very  disagreeable 
odor,  that  it  unites  readily  with  metallic  elements  and  forms 
salts.  Fluorine,  bromine,  and  iodine  also  are  all  possessed 
of  a  most  disagreeable  odor.  They  all  combine  easily  with 
metals  and  give  salts  which  strongly  resemble  the  chlorides. 
Whenever  the  chloride  of  a  metal  is  colorless,  the  fluoride, 
bromide,  and  iodide  of  that  metal  are  apt  to  be  also  colorless. 

377 


378     THE   HALOGEN  FAMILY;   THE   PERIODIC   SYSTEM 

Most  of  these  salts  are  soluble  in  water.  The  hydrogen  com- 
pounds are  all  soluble  in  water  and  form  acids,  namely  hydro- 
fluoric acid,  hydrochloric  acid,  hydrobromic  acid,  and  hy- 
driodic  acid.  All  four  elements  possess  the  same  valence  in 
their  compounds  with  metals  and  with  hydrogen,  namely, 
the  valence  one.  Hence  they  can  mutually  replace  each  other, 
atom  for  atom,  in  their  compounds  without  materially  alter- 
ing the  nature  of  the  compound.  Thus  the  symbols  of  the 
acids  are:  HF  HQ  HRr  ffl 

and  of  the  sodium  salts  are  : 

NaF        NaCl        NaBr        Nal 

FLUORINE 

400.  Uncombined  fluorine  is  a  gas  like  chlorine.  It 
possesses  a  similar  color  and  odor,  the  color  being  a  paler 
yellow,  whereas  the  odor  is  even  more  offensive.  Fluorine 
is  a  more  active  element  than  chlorine;  it  is  in  fact  the 
most  energetic  non-metallic  element  known.  It  reacts  spon- 
taneously with  all  of  the  ordinary  substances  of  which  the 
vessels  used  in  preparing  and  collecting  gases  are  made,  and 
for  this  reason  it  defied  for  a  long  time  all  attempts  to  ob- 
tain it  in  the  uncombined  condition. 

It  reacts  vigorously  with  water,  setting  oxygen  free  and 
forming  hydrofluoric  acid.  It  combines  spontaneously  and 
with  intense  energy  with  most  of  the  elements,  including 
sulphur,  phosphorus,  and  the  metals.  Gold  and  platinum 
vessels  resist  it  to  a  sufficient  extent  so  that  they  may  be 
used  as  containers  in  preparing  it. 

Fluorine  has  a  strong  action  on  glass.  It  attacks  organic 
compounds,  such  as  wood,  rubber,  paraffin,  and  oil,  with 
violence,  so  that  they  are  often  inflamed.  Fluorine  enters 


FLUORINE  379 

into  chemical  combination  with  the  silicon  of  the  glass  and 
with  the  hydrogen  of  the  organic  substances. 

401.  Preparation.     Fluorine  being  the  most  active  non- 
metallic  element,  there  can  be  no  other  element  which  can 
displace  it  from  its  compounds,  after  the  manner  in  which 
the  more  active  oxygen  of  oxidizing  agents  can  displace  the 
less  active  chlorine  from  hydrochloric  acid : 

O  +  2  HC1  -*  H2O  +  C12. 

Our  only  recourse,  then,  is  to  drag  fluorine  apart  from  the 
element  with  which  it  is  in  combination  by  means  of  the 
electric  current. 

Moissan,  a  noted  French  chemist,  succeeded  in  1886  in  ob- 
taining uncombined  fluorine  by  passing  an  electric  current 
through  a  solution  of  potassium  fluoride  in  dry  liquid  hydro- 
gen fluoride  contained  in  a  platinum  U-tube.  Hydrogen  was 
evolved  at  the  negative  electrode,  fluorine  at  the  positive 
electrode.  The  fluorine  was  passed  through  platinum  tubes 
and  was  caught  in  platinum  vessels  by  displacement  of  air. 
It  is  perhaps  needless  to  say  that  the  preparation  of  fluorine 
has  not  often  been  repeated. 

402.  Occurrence.     Fluorine  is  found  in  nature  as  a  con- 
stituent of  certain  minerals,  the  most  important  of  which 
is  fluorspar    (CaF2).     Unfortunately,   the  .phosphate  rock, 
CasCPO^,  which  is  used  in  making  fertilizer,  often  contains 
some  calcium  fluoride ;  the  hydrofluoric  acid  evolved  when 
the  rock  is  treated  with  sulphuric  acid  is  a  menace  to  the 
health  of  the  workmen. 

403.  Hydrofluoric  acid.    The  most  important  compound 
of  fluorine  is  hydrogen  fluoride,  or  hydrofluoric  acid,  HF. 
This  is  prepared  much  after  the  manner  of  hydrochloric  acid  : 
Fluorspar  is  treated  with  concentrated  sulphuric  acid  in  lead 


380    THE   HALOGEN   FAMILY;   THE   PERIODIC   SYSTEM 

retorts.  On  distillation,  the  gaseous  hydrogen  fluoride 
passes  off,  and  this  is  passed  into  water  in  lead  bottles,  where 
a  solution  of  hydrofluoric  acid  is  obtained.  This  acid  is  put 
on  the  market  in  bottles  of  gutta  percha  or  wax  because  glass 
is  attacked  by  it. 

Hydrofluoric  acid  does  not  attack  rubber,  wax  and  paraf- 
fin as  does  uncombined  fluorine,  because  the  element  is  al- 
ready in  combination  with  hydrogen  in  the  acid ;  but  hydro- 
fluoric acid  does  attack  glass.  It  reacts  with  the  silicon 
dioxide,  which  is  one  of  the  principal  components  of  glass : 

4  HF  +  SiO2  ->  SiF4  +  2  H2O. 

Silicon  tetrafluoride,  SiF4,  is  a  gaseous  substance  and  es- 
capes from  the  sphere  of  action. 

One  of  the  most  important  uses  of  hydrofluoric  acid  is  for 
etching  glass.  The  aqueous  acid  dissolves  away  the  glass 
and  leaves  a  smooth  hollowed  surface,  but  gaseous  hydrogen 
fluoride,  although  it  acts  chemically  in  much  the  same  way, 
leaves  a  roughened  surface  which  has  the  appearance  of 
ground  glass.  The  object  to  be  etched  is  coated  with  wax  and 
the  design  is  drawn  in  the  wax,  making  bare  the  glass  in  the 
desired  places.  Then  the  whole  is  exposed  to  hydrogen 
fluoride  gas  and  the  design  is  etched.  The  graduations  on 
our  glass  measuring  vessels  are  usually  marked  in  this  manner. 

Hydrofluoric  acid  is  extremely  poisonous;  its  vapor  is 
dangerous  to  breathe,  and  even  small  drops  of  the  solution 
cause  painful  ulcerated  sores  on  the  flesh. 

BROMINE 

404.  Uncombined  Bromine  is  a  dark  red  liquid  which 
freezes  at  -  7°C.  and  boils  at  63°  C.  It  is  about  three  times 
as  heavy  as  water.  It  evaporates  very  easily  at  ordinary  tern- 


BROMINE  381 

perature,  and  when  a  bottle  is  partly  filled  with  liquid  bro- 
mine, the  upper  part  is  always  filled  with  red  bromine  vapor. 
The  odor  of  bromine  is  similar  to  that  of  chlorine  and  it  is 
fully  as  offensive, — in  fact,  its  name  is  derived  from  the  Greek 
word  meaning  stench. 

The  chemical  reactions  of  bromine  are  much  the  same  as 
those  of  chlorine,  except  that  they  are  generally  less  vigorous. 
If  powdered  antimony  is  thrown  into  a  jar  of  bromine  vapor, 
it  does  not  flame  up  spontaneously  as  it  does  when  thrown 
into  chlorine.  If,  however,  the  antimony  is  dropped  into  a 
little  liquid  bromine  in  a  test  tube,  it  shows  a  vigorous  reac- 
tion. 

If  a  mixture  of  hydrogen  and  bromine  vapor  is  passed 
through  a  heated  tube,  the  elements  combine  quietly  and 
form  hydrogen  bromine,  but  there  is  no  explosion  as  when  a 
mixture  of  hydrogen  and  chlorine  reacts  (see  page  169). 

405.  Occurrence  and  Preparation.  Wherever  chlorine 
occurs  in  nature,  bromine  almost  invariably  appears,  but 
only  in  extremely  small  amounts.  Thus  in  the  brine  from 
salt  wells  there  is  usually  a  small  amount  of  bromine. 
After  these  brines  have  been  evaporated  and  all  the  salt  pos- 
sible has  been  crystallized  out,  the  "  mother  liquor  "  still 
contains  all  the  bromine  as  sodium  bromide  (or  magnesium 
bromide) .  The  bromine  is  then  liberated  from  this  compound, 
much  as  chlorine  is  liberated  from  common  salt,  namely,  by 
the  addition  of  sulphuric  acid  and  an  oxidizing  agent : 

H2SO4  +  2  NaBr  ->  Na2SO4  +  2  HBr 
2  HBr  +  O  ->  H2O  +  Br2. 

The  bromine  is  distilled  off  and  condensed  as  the  dark  red 
liquid  of  commerce. 

It  has  been  said  that  chlorine  is  a  more  active  element 


382    THE  HALOGEN  FAMILY;  THE  PERIODIC   SYSTEM 

than  bromine.  Once  in  combination,  it  is  more  difficult  to 
get  it  out  of  combination.  It  is,  therefore,  quite  as  one 
would  expect,  that  when  a  bromide  is  treated  with  chlorine 
the  bromine  is  forced  out  of  combination  while  the  chlorine 
enters  in  its  place : 

C12  +  2  NaBr  -+  2  NaCl  +  Br2. 

Use  is  often  made  of  this  fact  in  the  manufacture  of  bromine. 
Chlorine  gas  is  blown  into  the  hot  colorless  sodium  bromide 
liquor,  where  it  is  absorbed,  while  the  red  vapors  of  bromine 
emerge  and  are  blown  along  to  the  condensing  tubes. 

406.  Hydrobromic   acid.     Hydrogen   bromide   is   a  sub- 
stance most  strikingly  like  hydrogen  chloride  in  all  its  prop- 
erties.    It  is  a  colorless  gas  of  an  irritating  odor.     It  is  ex- 
tremely soluble  in  water  and  in  aqueous  solution  is  known 
as  hydrobromic  acid,  an  acid  which  is  fully  equal  in  strength 
to  hydrochloric  acid. 

407.  Uses  of  Bromine  and  of  Bromides.     Bromine  is  of 
more  practical  use  than  fluorine,  although  it  is  not  nearly  so 
much  used  as  chlorine,  whose  compounds  are  more  abundant. 
Most  of  the  uses  of  free  bromine  depend  on  its  being  an  oxi- 
dizing agent,  but  a  somewhat  less  vigorous  one  than  chlorine. 
It  finds  use  in  the  preparation  of  many  of  the  coal  tar  dyes. 

Some  of  the  bromides  are  used  as  sedatives  in  medicine. 
Silver  bromide  is  the  light-sensitive  substance  of  most  photo- 
graphic plates  and  films  and  of  much  of  the  photographic 
paper.  Silver  bromide,  like  the  chloride  and  iodide,  is  ex- 
tremely insoluble  in  water. 

IODINE 

408.  Uncombined  Iodine  is  a  dark,  blue-black  solid  sub- 
stance which  is  usually  formed  in  thin  platelike  crystals. 


IODINE  383 

It  is  so  dark  colored  that  it  appears  opaque  when  seen  in  a 
mass,  but  in  thin  plates  it  is  found  to  be  as  transparent  as  an 
equally  deep  colored  piece  of  glass.  Like  bromine,  iodine 
is  very  volatile,  and  if  a  small  grain  of  it  is  warmed  in  a  test 
tube,  the  tube  becomes  filled  with  the  most  beautiful  violet 
vapor  of  iodine. 

Iodine  is  a  noteworthy  substance  on  account  of  its  great 
volatility ;  when  it  is  warmed  in  an  open  dish,  as  for  example 
on  a  watch  glass,  it  all  passes  directly  from  the  solid  state  to 
the  vapor  state  without  first  melting.  If,  however,  a  consid- 
erable amount  of  iodine  is  heated  in  a  narrow-necked  flask, 
it  melts  at  114°  C.  and  the  liquid  iodine  boils  at  184°  C. 

The  chemical  reactions  of  iodine  are  very  similar  to  those 
of  chlorine  and  bromine,  except  that  they  are  much  less 
vigorous  than  the  latter.  Iodine  can  without  difficulty  be 
made  to  combine  with  the  more  active  of  the  metals,  but  the 
combination  is  not  attended  with  evidence  of  a  violent  re- 
action as  is  the  case  with  chlorine  and  to  a  lesser  extent  with 
bromine. 

409.  Uses  of  Iodine.  Iodine  dissolved  in  alcohol  or  in  a 
solution  of  potassium  iodide  finds  extensive  use  in  medicine. 
Concentrated,  it  is  used  as  a  counter  irritant,  —  being 
painted  on  the  skin  over  a  bruise  or  other  injury  to  bring  the 
blood  more  abundantly  to  the  surface  and  hence  relieve  the 
congestion  beneath.  In  a  more  diluted  condition  iodine 
solutions  make  a  valuable  antiseptic  wash-for  wounds.  The 
iodides  are  also  used  in  medicine. 

Hydrogen  and  iodine  can  be  made  to  combine  to  a  slight 
extent  when  a  mixture  of  the  two  is  passed  through  a  heated 
tube,  but  the  combination  is  far  less  complete  than  that  of 
hydrogen  and  bromine.  Hydrogen  iodide  is  a  colorless  gas 
like  hydrogen  chloride  and  hydrogen  bromide;  like  them, 

B.  AND  W.  CHEM. 25 


384    THE   HALOGEN   FAMILY;   THE   PERIODIC   SYSTEM 

also,  it  is  very  soluble  in  water  and  its  solution  is  a  strong 
acid,  —  hydriodic  acid. 

410.  Occurrence  and  Preparation.     Although  it  is  a  well- 
known  and  a  frequently  used  element,  iodine  is  not  at  all 
abundant    in   nature.      It    is    found    in    extremely  minute 
amounts  in  the  ashes  of  seaweeds  and   in  Chili  saltpeter. 
It  is  obtained  from  the  mother  liquor  when  the  latter  is  crys- 
tallized, much  as  bromine  is  obtained  from  the  mother  liquor 
when   common   salt  is   crystallized    (see   page   381).      The 
quantity  of  iodine  in  the  earth  is  about  as  many  times  less 
than  that  of  bromine  as  that  of  bromine  is  less  than  that  of 
chlorine. 

411.  Displacement   by   other   Halogens.     Iodine   is   the 
least  active  of  the  halogens ;   it  shows  the  least  energy  when 
it  enters  into  combination  with  metallic  elements,  it  is  corre- 
spondingly the  most  easy  to  separate  from  its  compounds. 
Both  chlorine  and  bromine  (and,  of  course,  fluorine)  are  able 
to  displace  it  from  its  compounds, 

C12  +  2  KI  ->  I2  +  2  KC1, 
Br2  +  2  KI  ->  I2  +  2  KBr. 

Thus  if  a  little  chlorine  is  bubbled  into  a  colorless  solution 
of  potassium  iodide,  the  iodine  is  displaced  according  to  the 
foregoing  equation,  the  liquid  becomes  dark  colored  in  conse- 
quence, and  flakes  of  solid  iodine  are  precipitated  out  when 
enough  has  been  set  free  to  more  than  saturate  the  solution. 
If  the  above  equations  are  written  in  ionic  form : 

C12  +  2  K+  +  2  I"  ->  I2  +  2  K+  +  2  CT, 

it  is  seen  that  the  metal  ion  is  unchanged  and  may  be  can- 
celled from  the  equation : 

ci2  +  2 1-  ->  2  cr  + 12. 


PERIODIC   CLASSIFICATION  OF  THE   ELEMENTS      385 

The  reaction  appears  thus  to  be  merely  a  matter  of  the  more 
active  negative  element  withdrawing  electric  charges  from 
the  less  active  element. 

Iodine  has  a  very  characteristic  property  of  giving,  with 
starch,  an  intense  blue  color.  When  so  small  an  amount 
of  iodine  is  set  free  in  solution  that  its  own  color  is  scarcely 
perceptible,  if  a  little  cooked  starch  paste  is  added,  a  marked 
blue  color  may  be  seen. 

Combined  iodine  does  not  give  this  test.  A  colorless 
mixture  of  potassium  iodide  and  starch  can  therefore  be  pre- 
pared ;  and  strips  of  paper  soaked  in  this  mixture  and  dried 
are  useful  in  testing  for  chlorine  and  bromine. 

THE  PERIODIC  CLASSIFICATION  OF  THE  ELEMENTS 

412.  In  the  living  world,  scientists  have  found  that  ani- 
mals and  plants  can  be  classified  into  families.  For  example, 
the  domestic  cat,  the  wild  cat,  the  leopard,  the  tiger,  and  the 
lion  are  placed  in  one  family  because  they  all  possess  certain 
family  characteristics  in  their  physiological  structure  and 
their  habits  of  life.  On  the  other  hand,  the  horse,  the  donkey, 
and  the  zebra  are  placed  in  another  family  because  they  have 
family  characteristics  markedly  different  from  those  of  the 
cat  family. 

A  similar  sort  of  classification  can  be  applied  in  the  in- 
organic world,  and  even  the  elements  themselves  fall  into 
natural  groups  or  families  of  which  the  halogen  family  is  a 
striking  example.  The  Russian  chemist,  Mendelejeff,  first 
clearly  perceived  and  showed  that  all  the  elements  could  be 
arranged  according  to  a  definite  system,  —  which  we  know 
as  the  periodic  system. 

He  arranged  all  of  the  elements  in  a  regular  order,  starting 


386    THE   HALOGEN   FAMILY;   THE   PERIODIC   SYSTEM 


with  the  one  of  lowest  atomic  weight  and  proceeding 
in  the  order  of  increase  of  atomic  weight,  and  he  thus 
showed  that  certain  special  properties  recur  at  fairly  regu- 
lar intervals.  The  elements  in  which  these  properties 
recur  are  the  ones  which,  like  the  halogens,  fall  into  the 
same  family. 

The  lightest  known  element  upon  this  earth  is  hydrogen, 
to  which  we  give,  arbitrarily,  the  atomic  weighj;  of  one.  This 
element  seems  to  stand  rather  by  itself,  but  the  next  eight 
elements,  with  their  atomic  weights,  are  shown  in  the  follow- 
ing table : 


HELIUM 

LITHIUM 

BERYL- 

BORON 

CARBON 

NITROGEN 

OXYGEN 

FLUORINE 

(4) 

(7) 

LIUM  (9) 

(11) 

(12) 

(14) 

(16) 

(19) 

1       00 

1* 

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1  a 

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Si 

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^  1^ 

C^) 

£H     Q^ 

S 

S 

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o>  § 

V*     n  *  rH 

•-*-^.  oC  tJD 

•  r-j    .^H 

^  "fr 

o 

^_j 

^a 

33s 

a  12 

SI 

igl 

1—  1 

H§ 

Li20 

BeO 

B203 

C02 

N205 

H4C 

H3N 

H20 

HF 

It  is  seen  from  the  few  properties  enumerated  in  the  table 
that  these  elements  are  very  unlike  each  other  in  their 
physical  properties  and  in  the  kind  of  compounds  that 
they  form.  Next  to  helium,  the  absolutely  inert  gas,  comes 
the  very  active  metallic  element  lithium.  The  next  element 
is  a  metal,  but  not  as  active  a  one  as  lithium.  The  next  is 
not  a  metal,  although  it  is  not  a  pronounced  non-metal.  With 
each  successive  element,  the  non-metallic  properties  grow 


PERIODIC   CLASSIFICATION  OF  THE   ELEMENTS     387 


more  marked,  until  in  fluorine  we  find  the  most  active  of  all 
the  non-metallic  elements. 

The  next  element  beyond  fluorine  is  neon,  but  we  do  not 
place  this  to  the  right  of  fluorine,  but  rather  we  begin  a  new 
series,  and  place  neon  beneath  helium,  for,  like  the  latter,  it 
also  is  an  absolutely  inert  gas.  Next  comes  sodium,  which  is  a 
sister  element  to  lithium  and  therefore  fits  naturally  into  the 
position  below  the  latter. 

The  complete  series  of  the  .eight  elements  following  fluorine 
is  shown  in  the  next  table,  and  an  inspection  shows  at  once 
a  marked  similarity  in  every  case  between  the  elements 


NEON 

SODIUM 

MAGNE- 

ALUMIN- 

SILICON 

PHOSPHO- 

SULPHUR 

CHLORINE 

(20) 

(23) 

SIUM  (24) 

IUM  (27) 

(28) 

RUS  (31) 

(32) 

(35.5) 

+a 

^ 

02 

02 

02 

„ 

i 

o3 

o3 

-a 

+5 

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Na20 

MgO 

A1203 

Si02 
H4Si 

P205 
H,P 

S03 
H2S 

C1207 
HC1 

that  occupy  the  corresponding  places.  Sulphur  corresponds 
to  oxygen  in  position,  and  we  have  already  learned  that  sul- 
phur and  oxygen  are  interchangeable  atom  for  atom  in  the 
sulphides  and  oxides.  Chlorine  corresponds  to  fluorine,  and 
we  have  already  discovered  their  strong  family  resemblance 
in  the  first  part  of  this  chapter. 

The  next  element  to  chlorine  is  argon,  another  inert  gas 
which  falls  naturally  into  the  position  below  neon ;    next  is 


388    THE   HALOGEN  FAMILY;  THE   PERIODIC   SYSTEM 

potassium,  an  alkali  metal  which  falls  naturally  below  magne- 
sium. As  we  go  on  in  this  next  series,  however,  we  find  that 
the  seventh  and  eighth  elements,  chromium  and  manganese, 
respectively,  are  heavy  metals  and  do  not  correspond  in  prop- 
erties with  oxygen  and  sulphur,  fluorine  and  chlorine,  respec- 
tively. In  fact,  we  now  have  to  go  through  a  series  of 
eighteen,  instead  of  eight,  elements  before  we  come  to  bromine, 
the  next  member  of  the  halogen  family.  The  periods,  after 
the  first  two,  lengthen  out  to  eighteen  instead  of  eight  ele- 
ments ;  and  the  fourth  as  well  as  the  third  period  contains  this 
number  of  members  (if  we  are  willing  to  take  one  for  granted 
which  has  not  yet  been  discovered,  but  which  we  feel  sure 
must  exist  and  probably  will  some  day  be  found),  and  the 
eighteenth  element  beyond  bromine  is  iodine,  another  of  the 
halogen  family. 

This  brief  discussion  of  the  periodic  system  shows  that 
there  must  be  some  regular  law  according  to  which  the  ele- 
ments have  been  formed  —  perhaps  are  still  being  infinitely 
slowly  formed.  So  far  we  have  taken  up  the  study  of  about 
twenty  only  of  the  most  common  of  the  elements,  and  it  is 
therefore  impossible  to  discuss  in  detail  all  of  the  wonderful 
analogies  brought  to  light  in  the  periodic  system. 

A  complete  table  of  the  periodic  classification  is  given  on 
the  opposite  page,  and  it  is  possible  to  tell  a  great  deal  about 
the  properties  of  any  element,  of  which  we  have  no  other 
previous  knowledge,  simply  by  observing  the  position  it  oc- 
cupies in  the  table.  It  will  strongly  resemble  the  other  ele- 
ments in  its  family  and  particularly  those  lying  nearest  to  it. 

Thus  we  can  conclude  that  krypton  and  xenon,  Kr  and 
Xe,  which  appear  in  column  0,  are  inert  elements  like  argon, 
and  thus  are  capable  of  forming  no  compounds  whatever. 
Rubidium  and  caesium,  Rb  and  Cs,  in  Family  A,  Group  I, 


PERIODIC   CLASSIFICATION  OF  THE   ELEMENTS     389 

_* 


Qo 


l 


a 


si 


N      , 


82 


P 


390     THE   HALOGEN   FAMILY;   THE   PERIODIC   SYSTEM 

are  very  active  alkali  metals  like  sodium  and  potassium. 
Strontium,  barium,  and  radium,  Sr,  Ba,  and  Ra  of  Family  A, 
Group  II,  are  active  alkaline  earth  metals  like  calcium. 
Selenium  and  tellurium,  Se  and  Te,  of  Family  B,  Group  VI, 
are  non-metals  resembling  sulphur. 

The  periodic  classification  has  been  of  very  great  use  in 
inspiring  and  directing  search  after  new  elements.  It  is  also 
very  useful  because  of  the  means  which  it  affords  of  enabling 
us  to  remember  the  properties  of  the  elements  by  groups. 
Thus  if  one  knows  well  the  general  properties  of  the  halogen 
family,  he  has  to  remember  but  little  more  in  order  to  be  quite 
familiar  with  the  properties  of  each  member  of  that  family. 

SUMMARY 

The  halogen  family  of  the  elements  includes  fluorine,  chlorine,  bro- 
mine, and  iodine  which  show  marked  resemblances  to  each 
other  in  their  physical  and  chemical  properties  and  constitute 
a  natural  family  of  the  elements. 

Fluorine  is  the  most  active  of  the  halogens,  in  fact,  of  all  the  non- 
metallic  elements.  Hence  it  cannot  be  displaced  from  its 
compounds  by  any  other  non-metallic  element.  It  can  be 
prepared  only  by  electrolysis. 

Hydrofluoric  acid  resembles  hydrochloric  acid  in  its  properties. 
It  is  prepared  from  calcium  fluoride  in  the  same  manner  as 
hydrochloric  acid  is  made  from  sodium  chloride.  Its  most 
remarkable  property  is  its  ability  to  attack  glass  and  it  finds 
use  in  etching  glass. 

Bromine  resembles  chlorine  but  is  denser  (being  liquid  at  ordinary 
temperatures),  darker,  and  less  active.  Chlorine  displaces  it 
from  its  compounds. 

Hydrobromic  acid,  like  hydrofluoric,  resembles  hydrochloric  acid. 

Iodine  is  solid  at  ordinary  temperatures  but  readily  volatilizes  if 
warmed.  It  resembles  the  other  members  of  the  family,  but 
it  is  the  least  active  and  can  be  displaced  from  its  compounds 
by  any  one  of  the  others. 


QUESTIONS  391 

Hydriodic  acid  resembles  hydrochloric  acid,  but  it  is  less  stable. 

Periodic  Classification.  With  increasing  atomic  weights  of  the  ele- 
ments, the  properties  undergo  regular  cycles  of  changes. 
Given  properties  recur  periodically  at  the  same  point  in  each 
cycle.  The  credit  belongs  to  Mendelejeff  for  arranging  the 
elements  according  to  the  periodic  classification  in  which  the 
members  of  a  given  family  occupy  similar  positions  in  the 
succeeding  periods. 

Questions 

1.  In  what  respects  do  the  elements  of  the  halogen  family  re- 
semble each  other  most  strongly  ? 

2.  Explain  the  principal  use  of  hydrofluoric  acid. 

3.  Compare  the  behavior  of  each  of  the  halogens  with  hydrogen. 

4.  How  would  you  test  a  material  to  see  if  it  contained  free 
iodine?  combined  iodine? 

6.    How  would  you  test  a  sweet  potato  for  presence  of  starch? 

6.  Explain  the  test  given  by  the  potassium  iodide-starch  paper 
for  free  chlorine  or  bromine. 

7.  How  might  a  study  of  the  periodic  table  of  the  elements  show 
what  elements,  not  yet  discovered,  may  sometime  be  found  in  the 
earth?     Could  the  properties  of  such  elements  be  foretold? 

8.  What  weight  of  iodine  is  contained  in  1000  grams  of  potas- 
sium iodide  ? 

9.  What  weight  of  chlorine  is  necessary  to  just  displace  the  iodine 
from  1000  grams  of  potassium  iodide? 

10.  What  volume  of  chlorine  (standard  conditions)  corresponds 
to  the  weight  taken  in  Question  9? 

11.  How  large  a  flask  may  be  filled  with  pure  bromine  vapor  at 
atmospheric  pressure  if  5  c.c.  of  liquid  bromine,  sp.  grav.  =  3.19,  is 
poured  into  the  flask  which  is  then  immersed  in  boiling  water? 

12.  Write  equations  for  the  following  cases :  (a)  CaF2  +  H2S04 
(hot  concentrated) ;    (6)  Mn02  +  HBr  (see  chapter  on  chlorine) ; 

(c)  Cu  +  Br2 ;    (d)  CuO  +  HBr. 

13.  Write  ionized  equations  for  the  following : 

(a)  KI  +  C12 ;  (6)  KBr  +  AgN03 ;  (c)  CuBr2  +  C12 ; 

(d)  HI  +  Zn ;  (e)  HBr  +  NaOH ;  (/)  HBr  +  CuO. 


CHAPTER  XXXII 

REVERSIBLE   CHEMICAL  REACTIONS;     CHEMICAL 
EQUILIBRIUM 

413.  Reversible  Reactions.     It  has  probably  not  escaped 
the  attention  of  the  thoughtful  reader  that  many  of  the  chem- 
ical reactions  studied  in  the  foregoing  chapters  can  under 
certain  conditions  be  made  to  reverse  themselves,  that  is  to 
say,  the  products  of  the  ordinary  reaction  may  be  made  to 
interact  with  each  other  to  give  back  again  the  substances 
originally  started  with.     Reactions  which  are  thus  capable 
of  proceeding  in  opposite  directions  under  different  conditions 
are  known  as  reversible  reactions. 

414.  Reversal    effected    by   Varying    the    Temperature. 
When  mercury  is  heated  moderately  in  the  presence  of  oxygen 
or  air,  it  combines  with  oxygen,  and  the  red  oxide  is  formed  : 

2  Hg  +  02  -+  2  HgO. 

When,  however,  as  in  the  course  of  the  preparation  of  oxygen 
(page  29),  mercuric  oxide  is  more  strongly  heated,  it  decom- 
poses into  mercury  and  oxygen  : 

2  HgO  ->  2  Hg  +  02. 

If  it  is  desired  to  draw  attention  to  the  reversibility  of  a 
reaction,  the  equation  is  written  with  a  double  arrow,  one 
pointing  in  each  direction  : 

2  HgO  ^>  2  Hg  +  O2. 

It  is  often  the  custom  to  make  one  arrow  heavier  to  indicate 
the  direction  of  the  prevailing  reaction. 

392 


REVERSAL  EFFECTED   BY  TEMPERATURE  393 

Such  reactions  as  the  one  just  mentioned,  in  which  the 
direction  may  be  changed  by  change  of  temperature,  are  very 
frequent. 

It  may  seem  a  little  perplexing  that  moderate  heat  appears 
to  be  necessary  to  induce  mercury  and  oxygen  to  combine, 
although  a  more  intense  heat  causes  decomposition.  As  a 
matter  of  fact,  the  lower  the  temperature  the  greater  is  the 
tendency  of  mercuric  oxide  to  form  in  comparison  to  its 
tendency  to  decompose.  All  reactions  take  place  more 
rapidly  when  hot,  more  slowly  when  cold,  and  the  particular 
reaction  of  the  formation  of  mercuric  oxide  takes  place  so 
slowly  at  room  temperature  that  it  is  not  perceptible. 

Heat  is  a  form  of  energy  which  works  most  often  in  opposi- 
tion to  chemical  affinity.  The  affinity  between  mercury  and 
oxygen  is  not  extremely  great,  —  mercury  being  one  of  the 
least  active  metals,  —  and  only  a  rather  moderate  degree 
of  temperature  is  necessary  to  overcome  this  affinity  and  de- 
compose the  compound.  This  temperature  is  only  a  little 
higher  than  the  temperature  necessary  to  make  the  reaction  of 
formation  perceptible,  and  thus  we  explain  the  fact  that  mer- 
cury and  oxygen  are  apparently  indifferent  to  each  other  at 
room  temperature,  that  they  unite  at  a  slightly  elevated 
temperature,  but  that  they  break  apart  again  at  a  still  higher 
temperature. 

What  has  been  said  of  mercuric  oxide  is  true  of  almost 
every  chemical  compound.  Take,  for  example,  water :  At 
ordinary  temperatures,  hydrogen  and  oxygen  gases  do  not 
react  perceptibly  when  mixed  together.  Raise  the  tempera- 
ture, however,  to  a  point  where  the  reaction  commences  to 
be  appreciable,  and  a  very  violent  explosion  ensues.  The 
water  vapor  formed  is  ordinarily  a  very  stable  compound, 
but  raise  it  to  2000°  C.  (a  temperature  higher  than  that  of 


394  REVERSIBLE   CHEMICAL   REACTIONS 

the  hottest  part  of  the  Bunsen  flame)  and  it  is  decomposed 
to  a  slight  degree  into  hydrogen  and  oxygen ;  raise  the  water 
vapor  to  3000°  C.  and  it  is  entirely  decomposed. 

Very  few  compounds  exist  that  cannot  be  decomposed  into 
their  elements  by  a  sufficiently  high  degree  of  heat.  Cal- 
cium and  oxygen  have  a  tremendous  affinity  for  each  other, 
and  calcium  oxide  can  withstand  the  temperature  of  the 
oxyhydrogen  flame  (about  2500°  C.)  without  decomposition. 
In  the  sun,  however,  where  the  temperature  is  estimated  to  be 
about  6000°  C.,  calcium  oxide  does  not  appear  to  exist,  for  the 
spectroscope  shows  us  the  presence  of  uncombined  calcium 
vapor. 

415.  Reversal  caused  by  Change  of  Concentration.  It  is 
possible  to  change  the  direction  of  chemical  reactions,  not  only 
by  altering  the  temperature,  but  by  varying  the  concentration 
of  one  or  more  of  the  reacting  substances.  The  effect  of  con- 
centration is  well  illustrated  by  a  process  which  was  much 
used  in  the  commercial  preparation  of  oxygen  before  the 
liquid  air  method  was  perfected,  and  which  is  based  on  the  re- 
versibility of  the  reaction  between  barium  oxide  and  oxygen 
on  the  one  hand,  and  barium  peroxide  on  the  other  hand : 

2  BaO  +  O2  ;±  2  Baft. 

Although  it  is  true  that  barium  oxide  will  take  on  oxygen 
from  the  air  at  a  moderate  red  heat,  whereas  barium  peroxide 
will  decompose  into  barium  oxide  and  oxygen  at  a  white  heat 
and  the  direction  may  thus  be  reversed  by  changing  the  tem- 
perature, it  was  found  in  practice  to  be  more  economical  to 
maintain  a  constant  temperature  at  700°  C.  and  to  effect  the 
change  by  altering  the  concentration  of  the  oxygen  in  the 
tubes  containing  the  solid  oxide. 

According  to  this  process,  air  is  first  forced  under  high 


CHEMICAL  EQUILIBRIUM  395 

pressure  into  the  tubes  and  the  concentration  of  oxygen  is 
thus  increased,  thereby  causing  the  oxygen  to  combine  with 
barium  oxide  and  form  the  peroxide.  The  nitrogen  which  is 
left  is  allowed  to  escape  through  a  valve  at  the  further  end 
of  the  tubes,  and  fresh  air  is  continually  pumped  into  the 
tubes.  When  all  of  the  barium  oxide  is  changed  to  barium 
peroxide,  the  pumps  are  reversed  so  as  to  produce  a  vacuum 
in  the  tubes.  The  concentration  of  the  oxygen  is  thus  re- 
duced and  the  barium  peroxide  begins  to  decompose  and  con- 
tinues to  give  off  oxygen  until  only  the  original  barium  oxide 
is  left.  The  oxygen  thus  pumped  out  is  forced  under  pres- 
sure into  the  steel  cylinders  in  which  it  is  marketed,  and  the 
barium  oxide  left  in  the  tubes  is  ready  to  be  again  carried 
through  the  same  cycle  of  operations. 

416.  Chemical  Equilibrium.  Barium  peroxide  has  a  tend- 
ency to  decompose  at  700°  C.  and  we  can  actually  observe 
that  it  gives  off  oxygen  at  this  temperature  into  a  vacuum 
or  into  a  space  containing  very  little  oxygen.  A  theory  of 
reversible  reactions  and  of  chemical  equilibrium  has  been 
worked  out ;  and  in  the  light  of  this  theory,  we  believe  that 
at  700°  C  barium  peroxide  decomposes  and  gives  off  oxygen 
just  as  rapidly  into  a  space  already  filled  with  oxygen  as  into  a 
vacuum.  Only  we  cannot  observe  it  under  these  conditions, 
for  the  reverse  reaction,  that  is,  the  formation  of  barium  per- 
oxide, is  now  progressing  more  rapidly  than  its  decomposition. 

When  no  oxygen  is  present,  this  reverse  reaction  cannot 
take  place,  and  we  observe  only  the  decomposition  of  the 
peroxide.  When  oxygen  is  present,  it  reacts  with  the  barium 
oxide  and  with  a  rapidity  that  depends  on  its  concentration. 
So  if  there  is  but  little  oxygen  in  the  chambers  with  the  barium 
oxide,  the  rapidity  of  the  combination  is  not  great.  The 
decomposition  of  the  barium  peroxide  is  progressing  all  the 


396  REVERSIBLE   CHEMICAL  REACTIONS 

time,  but  the  recombination  (which  we  regard  as  altogether 
another  reaction)  is  inappreciable.  So  when  the  exhaust 
pumps  are  working  and  the  amount  of  oxygen  is  being  kept 
low,  we  observe  only  the  decomposition  of  the  barium  perox- 
ide, which  is  then  the  predominating  reaction. 

If  the  amount  of  oxygen  in  the  chambers  is  increased, 
the  rapidity  of  the  recombination  is  increased.  When  a 
point  is  reached  at  which  the  rapidity  of  recombination  is 
equal  to  the  rapidity  of  decomposition,  the  two  opposed  reac- 
tions balance  each  other,  because  the  sum  of  their  effects  is 
zero.  No  change,  therefore,  can  be  observed  to  take  place, 
although  we  believe  that  the  two  opposite  reactions  are  really 
taking  place. 

If  the  amount  of  oxygen  in  the  chambers  is  increased 
still  further,  the  reaction  of  combination  begins  to  outdis- 
tance that  of  decomposition,  and  the  amount  of  barium  per- 
oxide increases.  This  is  the  condition  that  prevails  in  the 
process  when  air  is  pumped  under  high  pressure  into  the 
chambers  in  order  to  build  up  the  barium  peroxide. 

The  point  at  which  the  rate  of  the  opposed  reactions  is 
equal  and  at  which  there  is  therefore  no  apparent  change  is 
known  as  the  point  of  equilibrium.  Such  an  equilibrium  point 
exists  for  every  reversible  reaction,  and  if  a  system  is  left  to 
itself  either  one  reaction  or  the  other  will  predominate  until 
the  equilibrium  point  is  reached,  after  which  the  two  reactions 
will  continue  to  mutually  undo  the  effects  of  each  other. 

417.  Another  Reversible  Reaction  and  its  Equilibrium 
Point.  Let  us  consider  one  more  example  of  an  easily  revers- 
ible reaction  and  let  us  also  consider  the  conditions  which  lead 
to  a  state  of  equilibrium  when  the  rate  of  the  forward  reac- 
tion is  just  equaled  by  the  rate  with  which  the  products  of 
this  reaction  interact  to  form  again  the  original  substances. 


EQUILIBRIUM  OF  A   REVERSIBLE  REACTION         397 

It  will  be  recalled  that  one  of  the  practical  methods  of  pre- 
paring hydrogen  gas  is  to  pass  superheated  steam  through 
a  tube  containing  iron  turnings  kept  at  a  high  temperature. 
The  oxide  of  iron,  Fe3O4,  is  formed  and  hydrogen  is  left  un- 
combined. 


The  hydrogen  passes  out  of  the  further  end  of  the  tube  to- 
gether with  a  large  amount  of  undecomposed  steam.  The 
latter  condenses  to  liquid  water  when  it  is  cooled,  whereas 
the  hydrogen  passes  along  for  use. 

It  is^,  well-known  fact  that  iron  oxide  may  be  reduced  to 
metallic  iron  if  it  is  heated  in  a  tube  through  which  hydro- 
gen is  being  passed  : 

Fe3(V+  4  H2  ->  3  Fe  +  4  H2O. 

The  water  vapor  so  formed  is  swept  along  by  the  current  of 
hydrogen  and  drops  of  liquid  water  may  be  seen  to  condense 
beyond  in  the  cool  part  of  the  tube. 

The  two  reactions  just  discussed  are  the  exact  reverse  of 
each  other.  In  the  first  of  them,  an  abundant  supply  of 
steam  is  kept  continually  in  contact  with  the  iron.  This  nat- 
urally tends  to  react,  but  the  product  of  this  reaction,  the  hy- 
drogen, is  not  allowed  to  remain  to  reduce  the  iron  oxide  again, 
but  is  swept  along  out  of  the  way  by  the  excess  of  the  steam. 

In  the  second  of  the  reactions,  the  reverse  is  true.  Hy- 
drogen is  supplied  abundantly  and  has  every  opportunity  to 
react  with  the  iron  oxide.  The  water  vapor  formed  is  given 
no  chance  to  oxidize  back  any  of  the  reduced  iron,  for  it  is 
swept  along  out  of  the  way  by  the  excess  of  hydrogen. 

In  neither  of  these  cases  is  there  any  chance  given  to  at- 
tain an  equilibrium,  for  one  of  the  reaction  products  is  in  each 
case  removed  as  fast  as  it  is  formed. 


398  REVERSIBLE   CHEMICAL  REACTIONS 

If,  however,  iron  filings  are  heated  in  a  sealed  bulb  with 
some  water  vapor,  the  hydrogen  formed  remains  in  the  field 
of  action  and  it  begins  at  once  to  reduce  the  oxide.  As  the 
amount  of  hydrogen  increases,  the  rate  at  which  it  reduces  the 
iron  oxide  increases,  and  finally  a  point  is  reached  at  which 
the  rate  of  reduction  is  just  equal  to  the  rate  of  oxidation 
of  iron.  This  is  the  equilibrium  point  for  the  reversible 

reaction:          3  Fe  +  4  H2O  ^±  Fe3O4  +  4  H2. 

If,  on  the  other  hand,  we  start  with  iron  oxide  and  pure  hy- 
drogen in  the  bulb,  the  right-to-left  reaction  begins  at  once, 
but  as  the  water  vapor  accumulates,  the  rate  of  the  left-to- 
right  reaction  increases  until  equilibrium  is  reached  from  this 
side. 

The  point  of  equilibrium  for  this,  or  any,  reaction  is  ex- 
actly the  same  from  whichever  side  it  is  approached. 

418.  The  Mass  Law.  The  primary  cause  of  all  reactions 
is  chemical  affinity,  that  is,  the  mysterious  force  of  attrac- 
tion which  holds  compounds  together ;  but,  as  we  have  just 
seen,  the  direction  of  a  reaction  and  its  point  of  equilibrium 
are  strongly  influenced  by  two  other  factors,  namely,  tempera- 
ture and  concentration  of  the  reacting  substances  and  re- 
action products. 

At  a  constant  temperature,  the  course  of  many  reactions 
can  thus  be  determined  by  fixing  the  concentration  of  the  sub- 
stances involved.  The  term  "  mass  "  is  used  a  great  deal  in 
this  connection  instead  of  the  term  concentration,  and  the 
principle  just  stated  is  known  as  the  mass  law.  It  is  a  law  of 
the  most  far-reaching  importance  and  to-day  no  efficient 
chemist  undertakes  to  carry  through  a  chemical  process 
without  studying  it  fully  from  the  viewpoint  of  the  mass 
law. 


FORMATION  OF  PRECIPITATES  399 

Some  of  the  applications  of  the  mass  law  in  complex  mix- 
tures become  very  involved,  but  in  its  essence,  the  law  is  as 
simple  as  the  law  of  mass  plays  on  a  football  field,  —  that  the 
side  which  gets  the  most  men  and  the  strongest  men  into  the 
action  succeeds  in  pushing  the  ball  towards  the  opponents' 
goal. 

419.  Formation  of  Precipitates.  When  solutions  of  two 
salts  are  mixed  together,  there  is  oftentimes  very  little  evi- 
dence of  reaction.  For  example,  if  a  solution  of  potassium 
chloride  is  poured  into  a  solution  of  sodium  nitrate,  both 
salts  are  ionized : 

KC1  ^±  K+  +  Cr ;  NaN03  ^±  Na+  +  NOT. 

A  glance  at  these  formulas  shows  that  reactions  to  form  two 
new  salts, 

K+  +  NOT  ->  KNO3 ;   Na+  +  Cl~  ->  NaCl, 

are  possible,  but  these  are  both  reversible  reactions  and  the 
two  new  salts  have  a  strong  tendency  to  remain  in  the  ionized 
condition  (see  page  307).  Consequently,  there  is  but  very 
little  reaction.  If,  however,  it  were  possible  to  remove  one 
of  these  new  salts  from  the  sphere  of  action  as  fast  as  it 
formed,  it  could  no  longer  enter  the  reverse  reaction  and  we 
should  find  the  forward  action  prevailing. 

When  a  salt  is  insoluble  and  falls  out  of  the  solution  in  the 
form  of  a  precipitate,  it  is  effectively  removed  from  the  sphere 
of  action.  Thus  when  potassium  chloride  and  silver  nitrate 
solutions  are  mixed,  the  reaction, 

Ag+  +  Cr  ->  AgCl, 

becomes  possible  because  silver  chloride  is  extremely  in- 
soluble ;  as  fast  as  it  is  formed,  it  drops  from  the  solution 

B.   AND  W.   CHEM. 26 


400  REVERSIBLE   CHEMICAL  REACTIONS 

as  a  white  precipitate.  There  being  practically  no  silver 
chloride  left  in  the  solution  to  dissociate  again  into  the  ions, 
the  reaction  in  this  direction  is  lacking  and  consequently 
the  reaction  in  the  direction  of  the  formation  of  the  salt  be- 
comes complete. 

The  foregoing  is  but  one  illustration  of  the  general  rule : 
When  ionized  substances  are  mixed  in  solution,  a  reaction  will 
take  place  if  any  one  of  the  possible  products  is  insoluble.  The 
reaction  ivill  run  the  more  nearly  to  completion  the  more  insol- 
uble the  new  compound  is. 

420.  Formation  of  Volatile  Products.  In  the  preparation 
of  hydrochloric  acid  (page  129),  concentrated  sulphuric  acid 
is  allowed  to  react  with  solid  sodium  chloride  and  the  reac- 

tlon  2  NaCl  +  H2S04  ->  Xa2SO4  +  2  HClf 

takes  place.  Now  the  only  reason  that  the  reverse  reaction 
does  not  take  place  is  that  hydrogen  chloride  is  a  gas  and  is 
not  soluble  in  the  concentrated  sulphuric  acid.  It  therefore 
escapes  from  the  sphere  of  action,  and  the  reaction  which  re- 
sults in  its  formation  proceeds  to  completion. 

It  is  also  a  general  rule  that  whenever  any  product  of  a  possi- 
ble reaction  is  volatile  and  can  escape  from  the  sphere  of  action, 
that  reaction  will  take  place. 

That  the  direction  of  the  above  reaction  is  dependent 
on  the  conditions  which  favor  the  escape  of  hydrogen  chlo- 
ride can  be  proved  by  observing  the  effect  of  altering  the  con- 
ditions. Hydrogen  chloride  we  know  to  be  extremely  soluble 
in  water ;  therefore  if  it  is  formed  by  a  reaction  carried  out 
in  a  water  solution,  it  will  not  escape  from  the  sphere  of  ac- 
tion. When  sulphuric  acid  is  added  to  a  solution  of  sodium 
chloride,  no  effect  is  noticed  because  the  reaction  comes  to  a 
standstill  almost  immediately.  It  may  even  be  shown  that 


UNDISSOCIATED   SUBSTANCES  401 

the  reaction  can  be  forced  in  the  opposite  direction  when  the 
reaction  products  are  brought  together  in  high  concentration, 
for  if  hydrogen  chloride  gas  is  passed  into  a  nearly  saturated 
solution  of  sodium  sulphate,  a  heavy  precipitate  of  sodium 
chloride  is  thrown  down, 

2  HC1  +  Na2SO4  ->  2  NaCl|  +  H2SO4. 

421.  Formation  of  Undissociated  Substances.     We  have 
already  learned  that  the  neutralization  of  an  acid  and  a  base 
consists  in  the  formation  of  un-ionized  water  from  the  hydro- 
gen ions  of  the  acid  and  the  hydroxyl  ions  of  the  base.     It 
is  a  general  rule  that  whenever  different  ionized  substances 
are  brought  together  in  solution,  if  any  possible  new  combina- 
tion of  ions  can  yield  a  substance  whose  ability  to  ionize  is 
small,  that  substance  will  form. 

The  neutralization  of  a  strong  acid  with  a  strong  base  is 
a  non-reversible  reaction.  For  example,  take  the  neutraliza- 
tion of  hydrochloric  acid  by  sodium  hydroxide.  Both  of 
these  substances  exist  mainly  in  the  form  of  ions  in  dilute 
solutions.  When  they  are  mixed,  the  H+  and  OH~  ions  unite 
completely  in  the  formation  of  undissociated  water : 

H+  Cr  +  Na+  OH"  ->•  H2O  +  Na+  CT. 

There  is  no  tendency  to  make  this  reaction  reverse,  for  the 
only  possible  undissociated  compound  on  either  side  of  the 
equation  is  water. 

422.  Neutralization   of   Weak  Acids   and   Weak  Bases. 
Acetic  acid  is  a  good  example  of  a  weak  acid,  since  in  a  mod- 
erately dilute  solution,  it  is  only  about  one  per  cent  ionized, 
that  is  to  say,  one  in  every  one  hundred  molecules  is  separated 
into  ions,  H(C2H3O2)  ^±  H+  +  (C2H3O2)-,  whereas  the  other 
ninety-nine  are  intact.     Ammonium  hydroxide  is  a  base  of 
almost  the  same  degree  of  ionization  as  acetic  acid. 


402  REVERSIBLE   CHEMICAL  REACTIONS 

Both  this  acid  and  this  base  display  a  slight  electrical  con- 
ductivity, but  only  slight,  for  they  cause  the  incandescent 
filament  to  glow  just  visibly  when  they  are  tested  separately 
in  the  conductivity  apparatus  described  on  page  301.  But 
if  the  acid  and  base  solutions  just  tested  for  conductivity 
are  poured  together  and  the  mixed  solution  is  tested,  it  causes 
the  incandescent  filament  to  glow  with  its  full  brilliancy, 
thus  showing  a  conductivity  about  equal  to  that  of  any  fully 
ionized  salt.  By  merely  mixing  two  solutions  of  weak  con- 
ductivity, we  obtain  one  which  conducts  strongly. 

But  this  apparent  magic  is  easily  explained  according  to 
our  knowledge  of  the  equilibrium  conditions  of  reversible 
reactions.  In  the  acetic  acid  solution,  the  few  ions  are  in 
equilibrium  with  the  undissociated  molecules  according  to 
the  reversible  reaction 

H(C2H302)  ±£  H+  +  (C2H302)+, 

and  a  like  condition  holds  in  the  solution  of  ammonium 
hydroxide, 


4+  +  OR-         . 

But  when  we  mix  the  solutions,  more  H+  and  OH~  ions  are 
brought  together  than  can  exist  in  equilibrium  with  water 
in  the  reversible  reaction 

H+  +  OH-  ^±  H20. 

Consequently,  these  ions  disappear  because  they  unite  to 
form  water.  But  this  destroys  the  equilibrium  which  existed 
between  the  undissociated  acid  and  its  ions  and  between  the 
undissociated  base  and  its  ions.  The  acid  and  base  dissociate 
further  to  produce  more  Ii+  and  OH~  ions,  respectively,  in  an 
attempt  to  reestablish  equilibrium,  but  equilibrium  cannot 
be  established  because  the  H+  and  OH~  ions  are  continuously 


A   WEAK  ACID   AND   A  WEAK  BASE  403 

removed  in  the  formation  of  water  as  fast  as  they  are  formed 
by  the  ionization  of  the  acid  and  base. 

If  equivalent  amounts  of  the  weak  acid  and  weak  base  are 
taken  at  the  outset,  they  neutralize  each  other  almost  as 
completely  as  do  strong  acids  and  bases,  but  the  reaction  is 
more  complicated.  In  addition  to  the  formation  of  water 
from  the  H+  and  OH~  ions,  the  progressive  ionization  of  the 
acid  and  base  takes  place.  The  negative  ion  of  the  acid 
and  the  positive  ion  of  the  base  thus  formed  remain  as  ions 
in  the  solution  —  as  the  highly  ionized  salt  —  and  it  is  these 
new  ions  that  account  for  the  very  great  increase  in  conduc- 
tivity. The  whole  course  of  the  reaction  may  perhaps  best 
be  shown  by  the  following  scheme  : 

H(C2H302)    ^±    H+  +  (C2H302)- 
NH4OH    ^±    OH'  +  NIV 

I 
H2O 

NEUTRALIZATION  OF  A  WEAK  ACID  AND  A  WEAK  BASE 

The  two  reactions  written  horizontally  are  ordinarily  reversible 
and  by  themselves  tend  far  more  strongly  to  run  towards  the  left. 
When,  however,  certain  of  their  products  are  almost  completely 
withdrawn  as  fast  as  they  are  formed,  as  by  the  formation  of  water 
in  the  vertically  written  reaction,  the  horizontal  reactions  are  able 
to  run  nearly  to  completion  towards  the  right.  The  other  products 
formed  in  the  horizontal  reactions  accumulate  in  the  solution  and 
form  the  ionized  and  highly  conducting  salt,  NH4+  (C2H302)~. 

423.  Incompleteness  of  the  Neutralization  of  Very  Weak 
Acids  and  Bases.  When  an  acid  or  base  is  very  weak,  that  is, 
much  weaker  than  acetic  acid  or  ammonium  hydroxide,  its  neu- 
tralization is  less  complete  but  otherwise  takes  place  in  the 
same  manner.  It  dissociates  progressively  as  fast  as  its  dis- 
sociation product,  the  H+  or  the  OH~  ion,  as  the  case  may  be, 


404  REVERSIBLE   CHEMICAL  REACTIONS 

is  removed  through  the  formation  of  water.  Although  water 
on  the  one  side  is  the  least  dissociated  of  all  the  substances 
concerned  in  the  reaction,  yet  the  acid  or  base  on  the  other 
side  is  very  little  dissociated.  According  to  the  rule  above 
stated  that  reactions  tend  to  run  in  the  direction  in  which 
they  will  form  un-ionized  substances,  we  have  two  tendencies 
pulling  in  opposite  directions.  Water  being  the  least  disso- 
ciated, the  whole  reaction  tends  more  strongly  in  the  direc- 
tion of  neutralization. 

Carbonic  acid  is  a  very  weak  acid.  As  its  formula  shows, 
its  molecules  contain  two  acid  hydrogens.  One  of  these 
hydrogens  is  given  off  far  more  easily  than  the  other,  but 
the  first  one  dissociates  less  easily  than  the  hydrogen  of  acetic 
acid.  About  one  tenth  of  one  per  cent  of  the  molecules  of 
H2CO3  ionize  into  H+  and  HCO3~  ions.  When  treated  with 
one  mole  of  sodium  hydroxide,  carbonic  acid  reacts  almost 
completely  in  the  direction  of  the  arrows  in  the  diagrammatic 
scheme  ^  H+  +  HCQr 


+  Na+ 

I 
H20 

REACTION  OF  CARBONIC  ACID  WITH  ONE  MOLE  OF  SODIUM 
HYDROXIDE 

Neutralization  is  practically  complete.  Negative  bicarbonate 
ions,  HC03~,  accumulate  and,  balanced  by  Na+  ions,  form  the  ionized 
salt  sodium  bicarbonate,  Na+  HC03~. 

The  ion  HCO3~  itself  acts  asan  acid,  HCO3~  ^±  H+  +CO3  , 
although  to  a  far  less  degree  than  the  original  H2CO3;  and 
if  a  second  mole  of  sodium  hydroxide  is  added,  the  reaction 
represented  in  the  following  scheme  takes  place,  although  it 
does  not  progress  to  completion  : 


HYDROLYSIS  405 


Na+  HC03-    ^±    Na+  +  H+  +  CO 

Na+ 


H2O 

REVERSIBLE  REACTION  OF  SODIUM  BICARBONATE  WITH 
SODIUM  HYDROXIDE 

Neutralization  is  only  partial  with  an  acid  so  weak  as  HC03~. 
To  indicate  this  fact,  the  arrows  are  made  to  point  in  both  directions. 

424.  Hydrolysis.  Hydrolysis  is  the  exact  reverse  of 
neutralization.  When  an  acid  and  a  base  are  brought  to- 
gether, neutralization  takes  place  to  a  greater  or  less  extent, 
water  and  a  dissociated  salt  being  formed  ;  but  when  a  pure 
salt  is  treated  with  water,  hydrolysis  takes  place  to  some  ex- 
tent if  either  the  acid  or  the  base,  or  if  both,  are  very  weak. 

Na+  +  Na+  +  CQT' 
H2O    ^±    OH~  +  H+ 

It 
HCOs~ 

HYDROLYSIS  OF  SODIUM  CARBONATE 

Water  ionizes  to  a  very  small  extent  into  H+  and  OH~  ions.  The 
H+  ions  are  continuously  removed  by  the  COs  —  ions  to  form  HCO3~ 
and  so  the  water  is  allowed  to  ionize  continuously.  The  OH~  ions 
accumulate  and  impart  the  alkaline  character  to  the  solution.  Be- 
fore the  hydrolysis  has  progressed  far,  it  is  brought  to  a  standstill 
by  the  opposed  reaction  of  neutralization,  as  indicated  by  the  re- 
versed arrows. 

Thus,  for  example,  if  we  bring  together  sodium  carbonate  and 
water,  we  observe  hydrolysis,  for  the  resulting  solution  is 
found  to  be  alkaline  to  the  taste  and  to  turn  red  litmus  blue. 
On  the  other  hand,  a  salt  of  a  strong  acid  and  a  strong 
base,  for  example  sodium  chloride,  is  neither  alkaline  nor 
acid.  Sodium  carbonate  dissociates  at  once  in  water, 


406  REVERSIBLE   CHEMICAL   REACTIONS 

Na2CO3  ->  2  Na+  -f  CO3~  ",  but  the  CO3  ion  cannot  exist 
by  the  side  of  even  the  few  H+  ions  coming  from  water 
without  reacting  to  form  HCO3~.  Removal  of  even  a  part 
of  the  few  ions  which  water  normally  yields  allows  water 
to  ionize  further,  and  since  the  new  H+  ions  combine  further 
with  CCV" "  ions,  the  reaction  of  hydrolysis  progresses  in 
this  manner  until  it  is  overcome  by  the  opposing  reaction 
of  neutralization. 

425.  Alkalinity  of  Salts  of  Very  Weak  Acids.     Many  salts 
of  weak  acids  and  strong  bases  are  of  great  service  on  account 
of  their  mildly  alkaline  reaction.     Sodium  carbonate  is  com- 
monly known  as  washing  soda  and  its  usefulness  in  cleansing 
is  determined  by  its  mild  alkalinity.     Hydroxyl  ions  have  the 
property  of  causing  grease  and  dirt  to  emulsify,  that  is,  to 
separate  into  innumerable  little  globules,  so  that  they  are 
readily  floated  off  with  water. 

Soap  is  a  sodium  or  potassium  salt  of  the  weak  organic 
acids  which  are  constituents  of  fats.  Hydrolysis  of  soap 
plays  an  important  part  in  the  cleaning  action,  although  it 
is  true  that  soap  is  a  much  more  effective  cleaning  agent 
than  solutions  of  sodium  hydroxide  or  of  sodium  carbonate 
of  an  equal  degree  of  alkalinity. 

Borax  is  the  sodium  salt  of  the  weak  boric  acid,  and  borax 
is  often  used  to  give  a  mildly  alkaline  reaction. 

426.  Acidity  of  Salts  of  Very  Weak  Bases.     Salts  of  very 
weak  bases  and  strong  acids  show  acid  characteristics  when 
dissolved    in    water.     For    example,   aluminium    sulphate, 
A12 (804)3,  is  the  salt  of  the  weak  base  A1(OH)3  and  the  strong 
acid  H2SO4.     Its  hydrolysis  produces  equivalent  amounts  of 
the  weak  and  inactive  base  and  the  strong  and  active  acid 

A12(SO4)3  +  6  H2O  ^±  2  A1(OH)3  +  3  H2SO4. 


EXTENT  OF   HYDROLYSIS  407 

Hence  the  solution  turns  blue  litmus  red.  Alum,  which 
contains  aluminium  sulphate,  is  sometimes  used  in  baking 
powder  on  account  of  its  slightly  acid  character. 

The  hydroxides  of  the  heavy  metals  are  all  very  weak 
bases.  Hence  the  salts  are  inclined  to  hydrolyze.  Thus 
solutions  of  such  salts  as  ferric  chloride,  FeCl3;  ferric  sul- 
phate, Fe2(SO4)3;  ferric  nitrate,  Fe(NO3)3;  tin  chloride, 
SnCl2 ;  mercuric  nitrate,  Hg(NO3)2,  —  all  possess  an  acid  char- 
acter and  are  able  to  turn  blue  litmus  red. 

427.  Extent  of  Hydrolysis.  The  hydrolysis  of  a  salt 
always  produces  equivalent  quantities  of  acid  and  base.  Hy- 
drolysis cannot  occur  at  all  if  the  acid  and  base  are  both 
moderately  strong,  but  if  one  is  weak,  its  weakness  deter- 
mines the  degree  of  hydrolysis.  Since  the  weak  one  is  un- 
dissociated,  it  is  itself  inactive  and  the  other  imparts  its  acid 
or  alkaline  character  to  the  whole  solution. 

If  the  acid  and  base  are  both  very  weak,  the  hydrolysis  is 
all  the  more  complete.  For  example,  aluminium  sulphide 
is  the  salt  of  the  weak  base  A1(OH)3,  and  the  weak  acid  H2S. 
It  can  be  made  in  the  dry  way  by  heating  together  aluminium 
and  sulphur,  2  Al  +  3  S -+ A12S3. 

When  it  is  treated  with  water,  hydrolysis  takes  place, 
A12S3  +6  H2O  -+  3  H2S  +  2  A1(OH)3, 

with  a  vigorous  effervescence  due  to  the  escape  of  hydrogen 
sulphide  and  the  formation  of  a  flocculent  insoluble  residue 
of  aluminium  hydroxide.  Hydrolysis  is' complete  in  this  case, 
but  no  marked  alkaline  or  acid  effect  is  noted  because  both 
the  acid  and  the  base  are  very  weak. 

The  useful  hydrolyzable  salts  are  all  composed  of  either 
a  weak  acid  and  a  strong  base  or  of  a  strong  acid  and  a  weak 
base.  The  extent  of  the  hydrolysis  is  rarely  larger  than  a 


408  REVERSIBLE   CHEMICAL  REACTIONS 

few  per  cent  of  the  salt,  although  it  varies,  of  course,  ac- 
cording to  the  weakness  of  the  acid  or  base.  For  example,  a 
solution  of  sodium  carbonate  is  of  about  the  same  degree  of 
alkalinity  as  a  solution  of  ammonium  hydroxide  of  equivalent 
concentration,  and  we  have  already  found  that  ammonium 
hydroxide  is  only  about  one  per  cent  ionized. 

428.  Uses  of  Hydrolyzable  Salts.  When  it  is  desired  to 
produce  for  practical  purposes  a  solution  of  mild  alkalinity 
or  acidity,  the  use  of  hydrolyzable  salts  has  very  great  ad- 
vantages over  the  use  of  very  small  amounts  of  strong  bases 
or  acids.  For  example,  suppose  the  housewife  were  to  use 
sodium  hydroxide  to  scrub  the  kitchen  floor.  Knowing  little 
of  chemistry,  she  might  take  too  much,  and  the  skin  would  be 
taken  from  her  hands  and  the  floor  surface  would  be  injured. 
Washing  soda  gives  only  a  very  small  per  cent  of  sodium 
hydroxide,  and  she  cannot  take  enough  to  give  more  than  the 
desirable  mild  alkalinity. 

Suppose,  however,  that  she  was  careful  to  take  just  the 
small  amount  of  sodium  hydroxide  necessary  to  give  the 
proper  alkalinity.  A  little  acid  on  the  floor,  as  for  example 
from  crushed  fruit  or  spilled  vinegar,  would  quickly  neutralize 
the  small  amount  of  base  and  destroy  the  alkalinity.  On  the 
other  hand,  if  she  used  washing  soda,  the  acid  would  be  neu- 
tralized, but  more  of  the  salt  would  at  once  hydrolyzeto  main- 
tain the  alkalinity. 

A  similar  condition  governs  the  use  of  acid  substances  in 
baking  powder.  Sulphuric  acid  would  be  out  of  the  question, 
but  alum,  which  hydrolyzes  to  give  a  little  sulphuric  acid,  is 
useful.  As  fast  as  the  sulphuric  acid  so  formed  is  used  up  in 
reacting  with  the  sodium  bicarbonate  of  the  baking  powder, 
the  alum  hydrolyzes  further  until  finally  it  has  yielded  its 
entire  store  of  sulphuric  acid. 


HYDROLYZABLE  SALTS  409 

Another  important  use  of  a  hydrolyzable  salt  is  in  the 
settling  of  muddy  water  for  municipal  use.  Sedimenta- 
tion alone  would  be  too  slow  a  process,  although  it  would 
help  greatly.  Aluminium  sulphate  is  therefore  added  to 
the  muddy  water,  whereupon  hydrolysis  results,  yielding  a 
precipitate  of  gelatinous  aluminium  hydroxide  and  sulphuric 
acid.  The  gelatinous  aluminium  hydroxide  in  settling  car- 
ries down  most  of  the  suspended  material  with  it,  leaving 
clear  water.  The  sulphuric  acid  which  also  results  reacts 
with  calcium  bicarbonate  (present  in  most  water  supplies) 
and  forms  calcium  sulphate,  which  is  harmless,  and  carbonic 
acid,  which  decomposes  into  water  and  carbon  dioxide. 

429.  Importance  in  Living  Organisms.  The  delicate 
regulation  of  acid  or  alkaline  qualities  that  is  possible  with 
hydrolyzable  salts  is  not  only  made  use  of  practically  in  the 
household  and  in  the  chemical  factory,  but  it  occurs  exten- 
sively in  nature.  It  is  essential  to  the  life  processes  that 
the  blood  of  men  and  animals  and  the  juices  of  plants  be 
maintained  at  the  exact  degree  of  alkalinity  or  acidity  re- 
quired by  the  organism,  and  the  regulation  is  "usually  effected 
by  the  presence  of  hydrolyzable  salts.  The  human  blood 
is  maintained  at  almost  the  exact  neutral  point  by  means 
of  a  mixture  of  several  hydrolyzable  salts,  important  among 
which  is  sodium  phosphate.  If  the  blood  varies  appreciably 
to  either  side  of  this  point,  death  will  almost  immediately 
ensue.  These  salts  constitute  a  reservoir  of  neutrality. 
Suppose  they  were  not  present  and  that4  a  person  took  lemon 
juice  (citric  acid)  into  his  stomach.  This  would  pass  into 
the  blood  and  rapidly  increase  its  acidity  and  cause  death. 
But  with  sodium  phosphate  present,  the  citric  acid  is  neutral- 
ized by  the  base  progressively  set  free  by  hydrolysis.  If 
considerable  alkali  is  taken  into  the  stomach,  the  blood  does 


410 


REVERSIBLE   CHEMICAL  REACTIONS 


not  become  alkaline,  but  its  neutrality  is  maintained  by  a 
similar  process. 

430.    In  the  following  tables,  some  of  the  important  hydro- 
lyzable  salts  are  summarized  : 


SALT 

FORMULA 
OF  SALT 

WEAK  ACID 

STRONG 
BASE 

REACTION  OF 
SALT  SOLUTION 

Washing  Soda 

Na2CO3 

HCOS- 

NaOH 

Distinctly     al- 

kaline 

Baking  Soda 

NaHCO3 

H2COs 

NaOH 

Barely       alka- 

line 

Borax 

Na2B4O7 

H2B407 

NaOH 

Mildly      alka- 

line 

Soap 

Na(Ci8H35O2)i 

H(Ci?H36O2) 

NaOH 

Mildly      alka- 

line 

( 

Na3P04 

HPO4— 

NaOH 

Strongly   alka- 

Sodium   Phos-l 

line 

phate 

Na2HPO4 

H2PO4- 

NaOH 

Mildly  alkaline 

I 

NaH2PO4 

H(H2PO4) 

NaOH  2 

Mildly  acid 

SALT 

o°FRSMAULTA            STRONG  ACID 

WEAK 
BASE 

REACTION  OF 
SALT  SOLUTION 

Alum 

K2SO4  •  A12(SO4)3 

H2S04 

Al(OH), 

Mildly  acid 

•24H2O 

Aluminium  sul- 

phate 

A12(SO4)3 

H2SO4 

Al(OH)i 

Mildly  acid 

Sal  Ammoniac 

NH4C1 

HC1 

NH4OH 

Neutral  to  litmus 

Ferric  Chloride 

FeCh 

HC1 

Fe(OH)3 

Acid 

Gold  Chloride 

AuCl3 

HC1 

Au(OH)3 

Strongly  acid 

SUMMARY 

Many  chemical  reactions  are  practically  complete  under  ordinary 
conditions,  that  is,  they  proceed  entirely  in  one  direction  until 
either  one  (or  all)  of  the  reacting  substances  is  used  up. 

Chemical  affinity  is  the  force  which  causes  substances  to  combine. 
This  force  cannot  be  altered  by  changing  exterior  conditions, 

1  This  is  the  formula  of  sodium  stearate,  which  is  one  of  the  many 
similar  salts  which  constitute  soaps. 

2  Insufficient  to  neutralize  acid. 


SUMMARY  411 

but  the  exterior  conditions  may  produce  forces  which  can 
overcome  this  chemical  affinity.  Thus  reactions  can  often 
be  made  to  run  in  the  opposite  direction  by  the  application 
of  intense  heat  or  of  an  electric  current. 

Effect  of  Mass.  A  number  of  reactions  are  capable  of  reversal  even 
under  ordinary  conditions.  Such  reactions  are  governed 
largely  by  the  mass  of  the  reacting  substances.  Increase  in  the 
mass  of  the  reacting  materials  causes  the  reaction  to  proceed 
farther.  Increase  in  the  mass  of  the  products  of  the  reaction 
causes  the  change  to  proceed  in  the  reverse  direction. 

Chemical  Equilibrium.  A  reversible  reaction  seeks  to  reach  a  state 
of  chemical  equilibrium  which  is  a  condition  of  apparent  rest. 
The  substances  on  one  side  of  the  reaction  equation  exist  in  the 
presence  of  the  substances  on  the  other  side.  Although  at 
(  the  point  of  equilibrium  there  is  no  apparent  change  taking 
place,  still  it  is  believed  that  both  of  the  opposed  reactions  are 
taking  place,  —  only  with  equal  velocities  so  that  the  total  of 
the  resulting  change  is  zero. 

Removal  of  Products  of  Reaction.  If  a  substance  involved  in  a  re- 
versible reaction  is  continually  removed  so  that  the  reaction 
cannot  reach  a  condition  of  equilibrium,  the  reaction  may 
thus  be  made  complete  in  this  direction. 

Substances  may  be  removed  from  the  sphere  of  action  by  precipi- 
tation or  by  volatilization.  Un-ionized  substances  are  practi- 
cally inactive  so  that  when  they  are  formed  the  effect  is  as  if 
they  were  withdrawn  from  the  sphere  of  action.  Hence  the 
general  rule  :  Whenever  a  reaction  in  solution  is  possible  which 
can  produce  a  precipitate,  a  volatile  product,  or  an  undis- 
sociated  substance,  that  reaction  will  take  place. 

Neutralization  and  Hydrolysis.  Neutralization  is  a  reaction  in 
which  the  undissociated  substance,  water,  is  formed.  Hy- 
drolysis is  the  exact  reverse  of  neutralization.  Hydrolysis 
can  take  place  by  the  action  of  water  with  the  salt  of  a  very 
weak  acid  or  a  very  weak  base. 

Some  of  the  hydrolyzable  salts  are  of  very  great  usefulness  on 
account  of  the  mild  degree  of  akalinity  or  acidity  of  their 
solutions. 


412  REVERSIBLE   CHEMICAL  REACTIONS 

Questions 

1.  Give  several  reactions  that  you  have  already  studied  which 
are  capable  of  being  reversed. 

2.  Tell  of  one  means  by  which  reactions  are  made  to  reverse 
themselves. 

3.  What  is  meant  by  the  term  equilibrium? 

4.  Give  three  methods  of  causing  a  reaction  that  is  reversible 
in  its  nature  to  proceed  to  completion  in  one  direction. 

5.  What  is  meant  by  hydrolysis  ? 

6.  Why  does  sodium  carbonate,  which  is  salt,  have  an  alkaline 
reaction  in  water  solution  ? 

7.  Name  three  substances  the  principal  uses  of  which  depend 
upon  their  ability  to  hydrolyze. 

8.  Apply  the  idea   of  equilibrium  to  the  reversible   reaction 
NaCl  ^  Na+  +  Cl~,  in  a  solution  of  common  salt.     How  would 
this  equilibrium  be  affected  (a)  by  adding  a  solution  of  silver  nitrate, 
(6)  by  evaporating  off  the  water? 

9.  Does  guncotton  exist  in  a  state  of  chemical  equilibrium? 
What  would  you  do  to  make  guncotton  come  to  a  state  of  equilib- 
rium ?     What  is  the  reaction  and  is  it  a  reversible  one  ? 


CHAPTER  XXXIII 
CHEMICAL  REACTIONS  AND   ENERGY  TRANSFORMATIONS 

IN  the  first  chapter  of  this  book  it  was  stated  that  one  of 
the  most  noticeable  characteristics  of  chemical  changes  is  the 
evolution  of  heat  and  often  of  light.  Not  all  changes,  it  is 
true,  develop  as  much  heat  as  the  oxidation  of  charcoal ;  in 
fact,  some  reactions  absorb  heat ;  it  is,  however,  true  with- 
out exception  that  all  reactions  are  accompanied  by  some 
kind  of  an  energy  change. 

431.  Different    Forms    of    Energy.     Energy    manifests 
itself  in  many  forms,  of  which  heat  is  only  one.     The  heat 
produced  by  a  coal  fire  can  be  made  to  run  a  steam  engine  in 
which  a  part  of  the  heat  is  transformed  into  mechanical  en- 
ergy.    This  in  turn  can  be  made  to  rotate  the  coils  of  a  dy- 
namo and  produce  electric  energy.     The  later  can  be  turned 
back  again  into  mechanical  energy  by  means  of  an  electric 
motor,  or  it  may  be  converted  into  heat  in  an  electric  stove, 
or  into  heat  and  light  in  an  incandescent  lamp,  or  into  chemi- 
cal energy  by  being  made  to  decompose  some  chemical  com- 
pound. 

432.  Chemical  Energy.     When  an  electric  current  is  forced 
to  pass  through  acidulated  water,  the  latter  is  decomposed 
into  hydrogen  and  oxygen.     The  energy  possessed  by  these 
two  elements  is  thereby  increased  by  an  amount  equal  to  the 
electric  energy  expended  in  decomposing  the  water. 

It  is  true  that  hydrogen  and  oxygen  remain  quiescent 
at  ordinary  temperature  for  an  indefinitely  long  time.  This 

413 


414  ENERGY  TRANSFORMATIONS 

energy  may  be  compared  to  the' energy  of  water  in  a  reservoir 
in  a  high  mountain  valley.  The  water  is  quiescent  so  long  as 
it  remains  in  the  reservoir;  but  if  the  gate  is  opened  which 
allows  it  to  flow  downhill  to  a  water  wheel,  its  stored-up 
energy  can  do  mechanical  work.  In  a  similar  way,  hydrogen 
and  oxygen  when  uncombined  possess  stored-up  energy 
which  may  be  set  free  by  starting  them  in  reaction  with  a  tiny 
flame  or  a  spark.  If  a  mixture  of  the  two  is  exploded,  the 
expansion  can  be  made  to  do  mechanical  work,  or  if  the  gases 
are  slowly  fed  to  each  other  and  burned,  the  heat  of  their 
union  can  be  used  in  heating  a  steam  boiler. 

433.  Exothermic  Reactions.  A  reaction  which,  like  the 
combination  of  oxygen  and  hydrogen,  gives  off  heat  as  it  pro- 
ceeds, is  known  as  an  exothermic  reaction  (meaning  to  give 
off  heat).  In  general,  all  reactions  which  proceed  of  them- 
selves when  once  started  are  exothermic,  and  the  amount  of 
heat  or  other  form  of  energy  given  off  is  a  measure  of  the 
tendency  which  the  reaction  had  to  take  place. 

It  is  fortunate  that  strongly  exothermic  reactions  such 
as  the  burning  of  coal  and  wood  do  not  commence  sponta- 
neously. The  energy  stored  up  in  wood  and  coal,  except  for 
slow  decay,  which  can  be  likened  to  the  leakage  of  energy, 
stays  stored  up  waiting  for  the  hand  of  man  to  call  it  into  use- 
ful action  by  the  application  of  the  kindling  flame. 

Most  chemical  reactions  that  are  carried  out  in  the  labora- 
tory are  exothermic,  although  many  yield  so  little  heat  or 
yield  it  so  slowly  under  the  conditions  of  the  experiment 
that  its  escape  is  scarcely  noticeable.  For  example,  when 
substances  in  solution  enter  into  reaction,  the  heat  produced 
cannot  be  expected  to  produce  incandescence ;  the  water 
may  be  warmed  in  some  cases  so  that  it  boils  vigorously,  but 
more  often  it  will  be  warmed  only  a  few  degrees. 


ALUMINOTHERMY 


415 


When  pure,  dry  substances  react  and  the  heat  of  the 
reaction  all  goes  to  warming  a  limited  amount  of  solid  or 
molten  material,  the  temperature  may  rise  very  greatly. 
Thus  a  mixture  of  zinc  filings  and  powdered  sulphur  rises  to 
incandescence  when  a  reaction  is  started. 

434.  Aluminothermy.  A  very  high  temperature  can 
be  obtained  by  the  reaction  of  granulated  aluminium  with 
iron  oxide  : 


2M+ 


+  g 


The  mixture  of  iron  oxide  and  aluminium  is  placed  in  a  refrac- 

tory crucible  and  a  little  fuse  powder  is  placed  on  top.     The 

fuse    powder     when 

ignited  produces  an 

intense     local     heat 

and  this   starts   the 

reaction  of  the  main 

mixture.       A    refer- 

ence to  the  potential 

series  of  the  metals 

(page     329)     shows 

that     aluminium 

stands  high    in    the 

series  and  therefore 

that     it     must     be     a       FlG'    74>  —  preParin6  Molds  about  Rail  Joints  for 

welding  with  Thermit. 

very    active    metal. 

It  takes  oxygen  away  from  the  iron  oxide,  and  the  heat  of 
formation  of  the  aluminium  oxide  so  far  exceeds  the  heat 
necessary  to  decompose  the  iron  oxide  that  a  large  excess  is 
left  to  heat  the  residual  iron  and  the  aluminium  oxide. 
This  heat  is  all  confined  within  the  crucible,  for  no  gaseous 
products  are  formed  to  carry  away  part  of  it.  The  iron 
and  aluminium  oxide  are  thus  not  only  melted,  but  are  raised 

B.  AND  W.  CHEM.  -  27 


416 


ENERGY  TRANSFORMATIONS 


to  a  temperature  —  about  3000°  C.  —  approaching  that  of 
the  electric  arc. 

This  process  represents  the  highest  temperature  which  can 
be  obtained  for  industrial  purposes  by  means  of  a  chemical 

reaction.  A  mixture 
of  iron  oxide  and 
aluminium  is  a  com- 
mercial product  and 
is  sold  under  the 
name  of  Thermit. 
One  of  the  impor- 
tant applications  of 
Thermit  is  in  the 
welding  together  of 
iron  and  steel  parts 
as,  for  example,  the 
parts  of  a  broken 
propeller  shaft  of  a 
steam  vessel  and  the 
rails  of  the  car  track.  The  crucible  is  so  arranged  that  the 
molten  iron  which  sinks  to  the  bottom  can  be  tapped  off 
and  run  into  a  mold  placed  around  the  joint  to  be  welded. 
The  iron  is  so  hot  that  it  melts  the  surfaces  of  the  broken 
ends,  which  thus  become  welded  to  a  perfectly  solid  joint. 
In  making  welds  in  large  pieces,  it  is  a  common  practice  to 
preheat  the  pieces  to  be  welded  with  a  blast  flame.  This 
prevents  conduction  and  loss  of  so  much  of  the  heat  con- 
tained in  the  welding  steel  which  comes  from  the  crucibles 
and  so  insures  a  more  perfect  weld. 

435.  Familiar  Exothermic  Processes.  The  most  exten- 
sive industrial  chemical  reaction  is  the  combustion  of  fuel 
for  the  production  of  heat,  light,  and  power,  and  in  this  case 


FIG.  75.  —  Crucibles  containing  Thermit  in  Place 
over  Molds.  The  rubber  hose  shown  delivers  a 
blast  for  the  preheating  process. 


EXOTHERMIC  AND  ENDOTHERMIC  COMPOUNDS   417 

the  energy  set  free  is  all  that  is  sought  from  the  reaction ; 
the  material  products  are  of  no  value,  and  the  residues,  such 
as  ashes  and  smoke,  are  a  source  of  annoyance  and  expense. 

Most  of  the  chemical  reactions  in  which  it  is  the  material 
products  that  are  sought  are  also  of  the  exothermic  type. 
For  example,  in  the  manufacture  of  sulphuric  acid,  the 
oxidation  of  the  sulphur  yields  heat  at  both  stages  (Chapter 
XXIX)  and  the  union  of  sulphur  trioxide  with  water  also 
yields  heat. 

The  reactions  that  take  place  in  animal  bodies  are  also 
exothermic  and  serve  to  warm  and  move  the  body,  while  the 
material  products  of  the  reaction  are  waste  and  have  to  be 
eliminated.  The  food,  which  is  the  fuel  with  which  the  body 
is  fed,  is  not  all  burned  at  once,  but  a  large  part  of  it  is  slightly 
altered  chemically  without  much  loss  of  energy  and  stored 
up  as  muscular  tissue  and  fat.  In  this  form  it  remains  tem- 
porarily, but  finally  it  suffers  combustion  like  the  rest  of  the 
material  assimilated  by  the  body. 

436.  Exothermic  and  Endothermic  Compounds.  If  heat 
or  other  form  of  energy  is  evolved  by  the  reaction  in  which  a 
compound  is  formed  by  the  combination  of  its  constituents, 
the  compound  is  said  to  be  exothermic.  Exothermic  com- 
pounds, such  as  water,  zinc  sulphide,  aluminium  oxide,  are 
stable,  that  is  to  say,  they  never  decompose  of  themselves  and 
can  only  be  decomposed  by  the  expenditure  of  energy  in  some 
form,  —  such  as  electrical  or  chemical. 

When  heat  or  other  form  of  energy  is  absorbed  in  the  forma- 
tion of  a  compound  from  its  constituents,  that  compound  is 
said  to  be  endothermic  (meaning  heat  is  absorbed).  Nitro- 
glycerine is  an  example  of  an  endothermic  compound.  In 
general,  such  compounds  are  not  stable  and  they  tend  to 
decompose,  thereby  giving  off  energy ;  in  fact  many  of  them 


418  ENERGY   TRANSFORMATIONS 

decompose  explosively  when  the  reaction  gathers  sufficient 
headway.  The  energy  given  off  when  nitro-glycerine  decom- 
poses is  used  in  blasting,  —  the  blasting  material  dynamite 
is  infusorial  earth  or  other  porous  substance  impregnated 
with  nitroglycerine. 

437.  Photosynthesis.  We  see  on  every  hand  examples 
of  exothermic  chemical  change,  the  burning  of  fuels,  the  decay 
of  wood,  the  consumption  of  foods,  and  the  majority  of  the 
thousands  of  chemical  reactions  employed  in  the  manufactur- 
ing industries.  In  all  of  these  changes,  chemical  energy  is 
converted  into  heat.  The  thought  naturally  arises,  will  not 
all  of  the  chemical  energy  become  exhausted,  or,  in  other 
words,  will  not  all  of  the  materials  which  can  react  exother- 
mically  become  used  up  ?  It  will  be  recalled,  however,  from 
the  chapters  on  carbon  and  particularly  from  the  section  on 
the  carbon  cycle  in  nature,  that  all  fuels  and  foods  contain 
carbon  as  one  of  their  chief  constituents.  The  carbon  dioxide 
formed  by  the  combustion  of  these  materials  does  not  remain 
long  in  the  atmosphere,  but  reacts  with  water  in  the  green 
leaves  of  plants,  whereby  sugars  are  built  up  and  oxygen  is 
released  again  to  the  atmosphere.  The  sugar  so  formed  may 
be  changed  in  the  plant  into  starch,  cellulose,  or  other  sugars, 
and  ultimately  the  wood  may  be  converted  into  coal,  so  the 
sugar  formed  in  the  green  leaves  is  the  parent  substance  of  all 
foods  and  fuels.  The  combustion  of  sugar,  or  any  food  or 
fuel,  to  form  carbon  dioxide  and  water  is  a  strongly  exothermic 
reaction.  Consequently,  carbon  dioxide  and  water  cannot  of 
themselves,  —  nor  even  with  the  aid  of  catalyzers,  —  react 
to  form  sugar  and  oxygen.  Energy  must  be  applied  to  compel 
this  change,  and  this  energy  is  supplied  by  sunlight.  The 
chlorophyll  of  the  green  leaves  does  not  cause  this  chemical 
change,  but  its  function  is  to  direct  the  action  of  the  light 


THE   CARBON   CYCLE  419 

energy,  just  as  the  function  of  a  harness  is  to  make  the  power 
of  a  horse  available,  or  the  function  of  a  water  wheel  and  dy- 
namo is  to  convert  the  energy  of  falling  water  into  usable 
electric  energy. 

Thus  the  energy  of  the  sun  is  the  great  source  of  the  chemi- 
cal energy  stored  up  in  foods  and  fuels.  The  building  up 
process  is  called  photosynthesis,  meaning  the  putting  to- 
gether through  the  agency  of  light. 

438.  Energy  Changes  and  the  Carbon  Cycle.  From  what 
has  just  been  said,  it  is  seen  that  not  all  of  the  sun's  energy 
goes  at  once  to  simply  warm  the  surface  of  the  earth.  Some 
of  it  first  traverses  a  circular  path,  entering,  so  to  speak,  into 
single  revolutions  of  the  carbon  cycle.  On  the  one  side  of 
the  cycle,  the  green  plants,  by  using  water  and  carbon  dioxide 
as  materials,  and  sunlight  as  energy,  are  storing  up  chemical 
energy  by  means  of  endo thermic  reactions.  On  the  other 
side,  man  is  supplying  his  needs,  both  external  and  internal, 
and  all  animals,  and  also  bacteria  and  fungous  plants,  which 
live  upon  other  plants  or  upon  animal  tissues,  —  all  these 
are  bringing  about  exothermic  reactions  by  which  chemical 
energy  is  released,  ultimately  becoming  heat,  and  the  simpler 
substances,  water  and  carbon  dioxide,  are  once  more  formed 
as  a  result  of  the  combustion  or  decay  that  takes  place. 

The  material  elements  of  the  cycle  are  ready  to  go  through 
another  revolution,  but  the  energy  necessary  to  carry  on  the 
cycle  must  again  be  obtained  from  the  sunlight.  The  form 
in  which  energy  is  given  off  at  the  end  of  each  cycle  cannot  be 
again  utilized  by  the  green  plants. 

It  is  a  law  of  physical  science  that  no  energy  can  be  lost, 
and  this  is  as  fundamental  as  is  the  law  that  matter  cannot 
be  destroyed.  All  forms  of  energy  tend,  however,  to  change 
into  heat.  Heat  energy,  where  there  are  differences  in  tern- 


420  ENERGY   TRANSFORMATIONS 

perature  (intensity),  can  be  partly  converted  into  other  forms, 
as  for  example  by  the  steam  engine,  but  uniformly  distributed 
heat  cannot  be  converted  into  useful  forms  of  energy.  Thus 
large  bodies  of  water  contain  enormous  quantities  of  uni- 
formly distributed  heat,  but  this  form  of  energy  cannot  be 
made  to  propel  vessels.  The  earth  is  all  the  time  losing  en- 
ergy by  the  radiation  of  heat  into  space,  and  this  loss  is  prob- 
ably a  trifle  greater  than  the  gain  from  the  energy  received 
from  the  sun. 

439.  Artificial  Preparation  of  Endothermic  Substances. 
As  already  stated,  reactions  naturally  tend  to  proceed  in  the 
direction  in  which  energy  is  evolved,  and  outside  of  photo- 
synthesis, comparatively  few  endothermic  processes  are  ob- 
served in  nature.  Yet  it  is  possible  by  bringing  forces  into 
play  in  the  right  manner  to  cause  artificially  the  formation 
of  endothermic  substances,  and  man's  ingenuity  has  devised 
many  ways  in  which  to  manufacture  useful  compounds  of 
this  type. 

Nitric  oxide,  NO,  is  strongly  endothermic.  When  it  is 
made  from  atmospheric  oxygen  and  nitrogen  by  the  electrical 
method  (page  71),  its  energy  is  derived  from  the  heat  of  the 
electric  spark.  Nitric  oxide  is  of  importance  mainly  from  the 
fact  that  nitric  acid  can  be  made  from  it.  Nitric  oxide  com- 
bines with  oxygen  and  water  by  an  exothermic  process  —  one 
which  therefore  takes  place  without  further  application  of 
energy  —  to  form  nitric  acid. 

Nitroglycerin.  From  nitric  acid  are  derived  practically 
all  of  the  explosive  substances  in  commercial  use.  Typical 
of  these  explosives  is  nitroglycerin,  which  is  made  from  nitric 
acid  and  glycerol,  according  to  the  following  reaction : 

C8H6(OH)3  +  3  HN03  -^  C3H5(N03)3  +  3  H2O. 


ENDOTHERMIC   COMPOUNDS  421 

One  of  the  products  of  this  reaction  is  the  highly  exothermic 
substance  water,  and  the  energy  liberated  by  its  formation  is 
in  large  part  added  to  the  energy  already  possessed  by  the 
nitric  acid  and  the  glycerol  in  building  up  the  energy  con- 
tent of  the  nitroglycerin.  Furthermore,  concentrated  sul- 
phuric acid  must  be  added  to  the  reaction  mixture  (see 
manufacture  of  guncotton,  page  370)  in  order  to  carry 
forward  the  reaction.  Water  and  sulphuric  acid  mix  with 
the  evolution  of  heat,  and,  although  we  do  not  write  any 
definite  chemical  equation  for  this  combination,  still  it  is  ob- 
viously a  strong  exothermic  process.  Its  energy  is  likewise 
available  for  the  building  up  of  the  energy  of  the  nitro- 
glycerin. 

Calcium  carbide  is  an  endothermic  compound,  and  it  is 
formed  in  the  electric  furnace  according  to  the  reaction : 

CaO  +  3  C  ->  CO  +  CaC2. 

The  formation  of  carbon  monoxide  is  exothermic,  and  the 
energy  thus  liberated,  as  well  as  that  of  the  intense  heat  of  the 
furnace,  is  available  to  add  to  the  energy  already  possessed  by 
the  calcium  and  carbon. 

Acetylene  is  made  by  the  action  of  water  on  calcium  car- 

CaC2  +  2  H20  ->  Ca(OH)2  +  C2H2. 

Calcium  hydroxide  is  an  exothermic  substance  and  its  forma- 
tion aids  in  the  formation  of  the  highly  endothermic  acetylene. 
Acetylene  is  so  highly  endothermic  that  "it  is  dangerously  ex- 
plosive even  when  unmixed  with  air.  It  can  ordinarily  be 
handled  and  burned  with  a  fair  degree  of  safety;  but  when 
once  its  decomposition  is  violently  started,  it  continues  with 
great  violence.  The  great  amount  of  heat  liberated  by  its 
decomposition  causes  a  sudden  and  violent  expansion  and  it  is 


422  ENERGY  TRANSFORMATIONS 

this  that  does  the  damage.  One  reads  occasionally  of  serious 
explosions  occurring  in  acetylene  factories. 

The  endothermic  character  of  acetylene  adds  to  its  value 
as  a  fuel,  for  not  only  is  the  heat  of  formation  of  carbon 
dioxide  and  water  yielded  by  its  combustion,  but  added  to  this 
is  its  own  very  considerable  heat  of  decomposition.  The 
acetylene  flame  is,  therefore,  much  hotter  than  the  ordinary 
gas  flame  and  on  that  account,  as  well  as  because  it  contains 
more  luminous  carbon  particles  (see  page  275),  it  is  far  more 
luminous.  The  oxyacetylene  flame  is  much  used  in  place 
of  the  oxyhydrogen  flame  (see  page  116)  on  account  of  the 
superior  degree  of  heat  it  produces. 

Ozone  is  a  more  active  form  of  oxygen  because  it  is  pos- 
sessed of  a  higher  energy  content.  In  the  reaction  by  which 
it  is  formed  from  ordinary  oxygen,  3  O2  — >  2  O3,  energy  is 
absorbed  from  the  electric  impulses  which  constitute  the  so- 
called  silent  electrical  discharge. 

The  formation  of  hydrogen  peroxide  from  water  and  oxygen 
involves  the  acquisition  of  energy  and  it  is  this  energy  which 
accounts  for  the  activity  of  hydrogen  peroxide.  Dilute 
solutions  of  hydrogen  peroxide  are  not  dangerous  but  the  pure 
substance  is  highly  explosive. 

440.  Theory  of  Explosions.  Whether  an  explosion  takes 
place  from  the  union  of  two  substances,  as,  for  example, 
hydrogen  and  oxygen,  or  from  the  decomposition  of  a  sub- 
stance like  nitroglycerin  or  acetylene,  the  theory  is  the  same. 
The  explosive  reaction  is  an  exothermic  one.  So  long  as  the 
temperature  is  low  and  no  shock  is  given,  the  reaction  does  not 
acquire  any  headway ;  but  when  it  once  commences  to  a  con- 
siderable extent,  its  own  progress  furnishes  heat  which  raises 
the  temperature.  The  rise  in  temperature  promotes  the 
speed  of  the  reaction  itself ;  thus  more  heat  is  furnished  and 


THE  STORAGE  BATTERY  423 

so  on,  and  in  an  inconceivably  short  space  of  time  the  reaction 
is  finished. 

Some  substances  explode  more  easily  than  others;  nitro- 
glycerin  goes  off  with  extreme  ease  and  is  one  of  the  most 
dangerous  explosives  to  handle.  Guncotton  is  far  safer, 
and,  in  fact,  it  can  usually  be  burned  in  the  open  air  without 
exploding.  Yet,  when  it  is  exploded  by  detonation,  its  power 
is  not  much  inferior  to  that  of  nitroglycerin.  Acetylene  can  be 
burned  with  safety  as  an  illuminant,  yet  when  it  is  detonated, 
it  is  an  explosive  of  a  good  deal  of  power. 

441.  The  Storage  Battery  is  an  arrangement  of  electro- 
chemical cells  in  which  electrical  energy  can  be  converted 
into  chemical  energy  by  causing  an  endothermic  reaction  to 
take  place.  When  charged,  the  battery  is  ready  to  give  back 
all  the  energy  it  received.  The  chemical  reaction  takes 
place  in  the  reverse  direction,  and  in  so  doing  it  causes  an 
amount  of  electricity,  equal  in  amount  and  opposite  in  direc- 
tion to  the  charging  current,  to  flow.  The  reversible  reaction 
which  takes  place  in  the  usual  type  of  lead  storage  battery  is 

2  PbSO4  +  2  H2O  ^±  Pb  +  PbO2  +  2  H2SO4. 
The  reversible  reaction  in  the  Edison  storage  battery  is 
substantially  as  follows : 

Fe(OH)2  +  2  Ni(OH)2  ^±  Fe  +  Ni2O3  +  3  H2O. 

SUMMARY 

Exothermic  reactions  are  accompanied  by  an  evolution  of  energy 
so  that  the  energy  content  of  the  product,  or  products,  is  less 
than  that  of  the  reacting  substances.  The  energy  released 
is  most  commonly  in  the  form  of  heat,  but  it  may  be  in  some 
other  form,  as,  for  example,  electrical  energy  or  light.  Most 
reactions  that  take  place  in  nature  and  the  majority  of  those 
that  are  carried  out  in  the  laboratory  and  in  manufacturing 
processes  are  of  the  exothermic  type. 


424  ENERGY  TRANSFORMATIONS 

Endothermic  reactions  are  those  in  which  the  energy  content  of  the 
products  is  greater  than  that  of  the  reacting  substances.  Very 
important  endothermic  reactions  take  place  in  the  green 
leaves  of  plants  and  some  of  the  energy  of  sunlight  is  thereby 
converted  into  stored-up  chemical  energy.  The  material 
products  which  are  stored  up  in  the  plants  are  carbohydrates, 
and  the  process  is  called  photosynthesis. 

The  chemical  energy  continually  being  stored  up  by  plants 
in  forming  carbohydrates  and  oxygen  from  carbon  dioxide  and 
water  is  continually  being  used  up  in  satisfying  the  wants  of 
men  and  animals,  and  in  very  large  amount  being  wasted  by 
decay  and  useless  fires. 

Exothermic  compounds  are  stable,  whereas  endothermic  com- 
pounds are  unstable  and  many  of  them  are  explosive. 

Endothermic  compounds  are  made  artificially,  sometimes  by  uti- 
lizing electric  energy,  sometimes  heat  energy,  and  sometimes 
the  energy  liberated  in  the  formation  of  exothermic  compounds. 

Questions 

1.  Cite  several  exothermic  reactions  which  are  carried  out  solely 
for  the  heat  produced  and  in  which  the  material    products  are 
merely  waste. 

2.  Cite  exothermic  processes  which  yield  energy  as  an  electric 
current. 

3.  How  do  chemical  processes  store  up  the  sun's  energy? 

4.  Name  several  explosives  in  which  the  power  is  developed  by 
the  decomposition  of  an  endothermic  substance. 

5.  Describe  explosive  mixtures  in  which  the  power  is  developed 
through  the  formation  of  an  exothermic  compound. 

6.  To  which  of  the  sciences  does  the  study  of  energy  naturally 
belong?     Why  is  it  impossible  to  study  any  one  science,  such  as 
chemistry,  without  overlapping  into  the  other  sciences? 

7.  Why  is  a  knowledge  of  chemistry  fundamental  in  the  under- 
standing of  the  sciences  of  physiology  and  geology? 


APPENDIX 

I.   PRESSURE  OF  SATURATED  WATER  VAPOR 

Temperatures  in  degrees  centigrade.     Pressures  in  millimeters  of 

mercury 


TEMP. 

PKESS. 

TEMP. 

PRESS. 

TEMP. 

PRESS. 

0 

4.6 

11 

9.8 

22 

19.7 

1 

4.9 

12 

10.5 

23 

20.9 

2 

5.3 

13 

11.2 

24 

22.2 

3 

5.7 

14 

11.9 

25 

23.6 

4 

6.1 

15 

12.7 

26 

25.0 

5 

6.5 

16 

13.5 

27 

26.5 

6 

7.0 

17 

14.4 

28 

28.1 

7 

7.5 

18 

15.4 

.  29 

29.8 

8 

8.0 

19 

16.4 

30 

31.6 

9 

8.6 

20 

17.4 

50 

92.0 

10 

9.2 

21 

18.5 

100 

760.0 

II.  CORRECTION  OF  GAS  VOLUMES  FOR  AQUEOUS 

VAPOR 

When  a  gas  is  saturated  with  water  vapor,  the  volume 
which  it  would  occupy  dry  may  be  calculated  with  the  aid  of 
the  above  table  and  the  following  two  rules. 

In  a  mixture  of  two  or  more  gases  each  gas  exerts  its  own 
pressure  independently  of  all  the  other  gases  present,  and  the 
total  pressure  is  equal  to  the  sum  of  tlie  pressures  which  each 
one  would  exert  if  it  were  the  only  gas  filling  the  space. 

The  amount  of  water  which  is  able  to  evaporate  into  a  defi- 
nite space  depends  solely  on  the  temperature  and  is  independent 
of  the  quantity  and  kind  of  other  gas  which  already  fills  the 
space. 

425 


426  APPENDIX 

If  a  little  water  is  admitted  to  an  evacuated  vessel  and  the 
temperature  is  20°  C.,  the  water  evaporates  until  the  pressure 
of  its  vapor  is  17.4  mm.  (see  foregoing  table).  If  the  vessel 
is  already  filled  with  dry  air  at  760  mm.  before  the  water  is 
admitted,  —  or  forced  in,  —  the  water  evaporates  just  the 
same  until  its  pressure  has  reached  17.4  mm.  A  barometer 
placed  in  the  vessel  would  now  show  a  pressure  of  760  +  17.4 
mm.,  provided  the  vessel  were  kept  closed  and  its  pressure 
were  not  allowed  to  equalize  itself  with  that  of  the  outside 
atmosphere. 

Suppose  we  were  measuring  the  volume  of  a  gas  by  confin- 
ing it  over  water  at  20°  C.  and  760  mm.  The  gas  is  saturated 
with  water  vapor,  the  pressure  of  which  at  20°  C.  is  17.4  mm. 
Since  the  total  pressure  is  only  760  mm.  and  the  gas  itself 
is  only  one  of  the  two  gases  which  constitute  the  mixture, 
the  pressure  of  the  gas  itself  can  be  but  760  —  17.4  =  742.6 
mm. 

In  the  problem  on  page  87  it  was  found  that  45  c.c.  of  dry 
gas  at  20°  C.  and  740  mm.  would  have  a  volume  of  40.8  c.c. 
under  standard  conditions.  Let  us  now  calculate  what  would 
be  its  volume  under  standard  conditions,  if  the  gas  had  been 
saturated  with  water  vapor  when  its  volume  was  measured : 

pressure  of  gas  +  pressure  of  water  vapor  =  740  mm. 
pressure  of  gas  =  740  —  pressure  of  water  vapor 
=  740  -  17.4  =  722.6  mm. 

v      stand,  temp.      press,  of  gas 

vol.  (standard)  =  vol.  (meas.)  X  -         — ^ X  - 

temp,  ot  gas      stand,  press. 

273    ,  722.6 
=  45X^3 

=  39.9  c.c. 


APPENDIX 


427 


This  volume  is  0.9  c.c.  less  than  the  volume  calculated  for 
the  dry  gas,  and  we  thus  see  that  at  20°  C.  saturating  a  gas 
with  water  vapor  increases  its  volume  by  2  per  cent  when 
the  total  pressure  is  not  allowed  to  increase. 

Therefore  in  making  accurate  measurements  the  presence 
of  water  vapor  should  not  be  neglected  when  calculating  gas 
volumes  to  standard  conditions. 

On  the  other  hand,  when  a  number  of  gas  volumes  are 
being  measured  over  water  all  at  about  room  temperature 
and  it  is  necessary  only  to  make  a  comparison  of  the  different 
volumes,  it  is  unnecessary  to  take  the  water  vapor  into  ac- 
count, because  it  is  present  in  every  case  in  almost  the  same 
proportion. 

In  correcting  to  the  same  conditions  the  volumes  measured 
in  determining  the  volume  per  cent  of  oxygen  in  air  (page  85) 
the  presence  of  water  vapor  was  neglected.  The  pupil  is 
advised  to  recalculate  this  problem,  correcting  all  volumes 
to  standard  conditions  (dry)  and  convince  himself  that  the 
same  per  cent  by  volume  of  oxygen  is  found. 

III.     SOME   PROPERTIES  OF  COMMON  ELEMENTS 


ELEMENT 

SPECIFIC 
GRAVITY 

MELTING  POINT 

BOILING  POINT 

2  7 

fi^7°  P 

Calcium      .... 

1.58 

C  Q 

800 
104.^ 



Gold       

19.3 

1065 

9^7 

OCOO  p 

Iron        

78 

1550 

Lead       
Mercury     .... 

11.4 
13.6 

01    K 

327 
-39.4 

i  77f) 

357 

Potassium  .... 
Silver 

.87 
105 

62.5 
955 

720 

428 


APPENDIX 


III.   SOME  PROPERTIES   OF   COMMON   ELEMENTS— 

Continued. 


ELEMENT 

SPECIFIC 
GRAVITY 

MELTING  POINT 

BOILING  POINT 

Sodium       .     .     .     . 
Tin 

.97 

7  3 

95.6 
233 

742 

Tunsrsten 

1Q  1 

flhnvp  ^000 

Zinc        
Bromine      .... 
(  diamond 
Carbon  -}  graphite 
(  amorphous 

7.2 

3.2  (liquid) 
3.51 
2.25 
1.70 

420 

7 

^3500? 

i  no 

918 

59 

00    K 

L\JA 
914 

—     oo.O 
1  Qfi 

Oxygen       .... 
Phosphorus  (yellow) 
Iodine    . 

1.82 
4  95 

-227 
44.2 
114 

-  181 
269 
184 

*W»  |  [1-SiO 

2.06 
1.96 

114.5 
119 

|449 

IV.   WEIGHT   OF   ONE   LITER   OF  VARIOUS   GASES 
UNDER   STANDARD   CONDITIONS 


GAS 

FORMULA 

WEIGHT 

Air 

1  29  grams 

Ammonia 

NH3 

0  76 

Argon 

A 

1  79 

Carbon  dioxide      
Carbon  monoxide  
Chlorine  . 

C02 
CO 

Clo 

1.98 

I'll     toV 

Fluorine  
Helium    
Hydrogen 

F2 
He 
H2 

1.70 
0.18 
0  09— 

Hydrogen  chloride     
Hydrogen  sulphide    
Methane 

HC1 
H2S 
CH4 

1.61' 
1.54 
072 

Nitrogen 

N2 

1  25 

Nitric  oxide      
Oxygen    
Steam  (hypothetical)      .... 
Sulphur  dioxide     

NO 

0, 
H20 

S02 

1.34 
1.43  — 
0.81 
2.86 

APPENDIX  429 

V.     SOME   PROPERTIES   OF  COMPOUNDS 


NAME 

FORMULA 

SPECIFIC 
GRAVITY 

STATE 

MELT- 
ING 
POINT 

BOILING 
POINT 

Acetic  acid  (glacial) 

H(C2H302) 

1.055 

liquid  above 

17.5°  C. 

119.0°  C. 

17.5°  C. 

Acetone 

C3H60 

0.79 

liquid 



56.3 

Alcohol  (grain) 

C2H6OH 

0.79 

liquid 

-130 

78.4 

Alcohol  (grain) 

95%  constant  boil- 

ing mixture 

C2H5OH  Aq 

0.80 

liquid 



78.15 

Alcohol  (wood) 

CH3OH 

0.79 

liquid 

-97.8 

66 

Ammonia 

NH3 

(    8.5(H2=1)    1 
\0.73(HjO=l)J 

/gaseousl 
\  liquid  / 

-75.5 

-33.5 

Ammonium 

(  NH4OH  } 

hydroxide 

I  NH3  in   \              0.90 

solution 





(28%  NHs) 

(  solution  J 

Carbon  dioxide 

C02 

/22.0  \ 
\  0.72J 

/gaseousl 
1  liquid  J 



-79 

Carbon  tetrachloride 

ecu 

1.59 

liquid 



77 

Chloroform 

CHCls 

1.52 

liquid 

-63.2 

61.2 

Ether 

(C2H5)20 

.73 

volatile 
liquid 

-117 

35 

Hydrochloric  acid 

(concentrated  ;  40%, 

HCI) 

HCI  Aq 

1.20 

solution 





Hydrochloric  acid 

(constant        boiling 

point        mixture; 

20.2%  HCI) 

HCI  Aq 

1.10 

solution 



110 

Hydrogen  chloride 

HCI 

(18.21 
1  0.8/ 

/gaseousl 
1  liquid  / 

-116 

-80 

Hydrogen  peroxide 

H202 

1.45 

i-       -i 
liquid 





Hydrogen  sulphide 

H2S 

/17.0  \ 
I     -86 

/gaseousl 
\  liquid  } 

-82.9 

-61.8 

Nitric  acid  (pure) 

HNOs 

1.52 

liquid 

-47 

86 

Nitric  acid 

(constant        boiling 

point          mixture 

(68%  HNOs)) 

HNOs  Aq 

1.42 

Solution 



120.5 

Petroleum  products 

Various  hy- 

(a)  Petroleum  ether 

drocarbons, 

0.635  to  0.660 

mixed  liquid 



40  to  70 

(6)  Gasoline 

chiefly  of  the 

0.660  to  0.667 

mixed  liquid 



70  to  80 

(c)    Benzine 

marsh       gas 

0.667  to  0.707 

mixedliquid 



80  to  100 

(d)   Kerosene 
Grade  I 
Grade  II 
Grade  III 

series.      The 
mean  molec- 
ular    weight 
increases 

0.753  to  0.864 

J 

mixed  liquid 



150  to  200 
200  to  250 
250  to  300 

with  increase 

in  the  boiling 

point. 

Phosphoric  acid 

H3PO4 

1-88 

solid  below 

40.5 



(ortho) 

40.5°  C. 

Sulphur  dioxide 

S02 

/32      } 

/gaseousl 
i  liquid  / 

-76.1 

-8 

Sulphuric  acid     (98% 

H2S04  • 

H2S04) 

i  H2O 

1.84 

liquid 

below  0 

383 

Sulphuric  acid  (mono- 

hydrate) 

H2SO4  •  H2O 

1.79 

liquid 

8 

. 

Sulphur  trioxide 

80s 

f  1.901 

11.97/ 

/liquid! 
\  solid  / 

14.8 

46 
50 

430 


APPENDIX 


VI.    SOLUBILITIES   OF  SOME  BASES  AND  SALTS 

Grams  of  Substance  Soluble  in  100  cc.  of  Water  at  18°  C. 


K 

Na 

Ag 

Ba 

Ca 

Mg 

Zn 

Pb 

Cl 

33 

36 

0.0316 

37 

73 

56 

204 

1.5 

Br 

66 

89 

o.oa 

104 

143 

103 

478 

0.60 

I 

137 

178 

0.0*3 

201 

200 

148 

419 

0.08 

N03 

30 

84 

213 

8.7 

122 

74 

118 

52 

C1O3 

6.6 

97 

12 

35 

179 

126 

184 

151 

SO, 

11 

36 

0.55 

0.0.23 

0.20 

35 

53 

0.004 

C03 

108 

19 

0.003 

0.0023 

0.0013 

0.1 

0.004  ? 

0.0*1 

OH 

149 

11C 

0.01 

3.7 

0.17 

0.001 

0.035 

0.01 

VII.     GENERAL   RULES   OF   SOLUBILITY 

In  addition  to  the  above  exact  figures  for  the  solubility  of  a  few 
substances,  the  following  approximate  rules  are  of  value  : 

1.  All  salts  of  the  alkali  metals  (Na,  K,  NH4)  are  soluble. 

2.  All  bromides  are  soluble  except  those  of  Ag,  Hg,  Pb. 

3.  All  carbonates  are  insoluble  except  those  of  the  alkali  metals. 

4.  All  chlorates  are  soluble. 

5.  All  chlorides  are  soluble  except  those  of  Ag,  Pb,  and  Hg  (ous) . 

6.  All  hydroxides  are  insoluble  except  those  of  the  alkali  metals 
(Na,  K,  NH4),  Ba,  and  Sr. 

7.  All  nitrates  are  soluble. 

8.  The  oxides  of  the  heavy  metals  are  insoluble  and  do  not  re- 
act with  water.     The  oxides  of  the  alkali  metals,  Ca,  and  Ba  react 
vigorously  with  water  to  form  the  hydroxides. 

9.  All  phosphates  are  insoluble  except  those  of  the  alkali  metals. 

10.  All  silicates  are  insoluble  except  those  of  the  alkali  metals. 
Glass  and  rocks,  such  as  feldspar  and  mica,  are  insoluble,  although 
they   contain   silicates    of    the    alkali   metals   mixed   with    other 
silicates. 

11.  All  sulphates  are  soluble  except  those  of  Ba,  Ca,  Sr,  and  Pb. 


APPENDIX 


431 


VIII.     IMPORTANT  TEMPERATURES 


2. 
3. 

Zero  absolute      ...  -  2 
Boiling  point  of  helium    — 
Melting  point  of  hydrogen 

73°  C. 

268.5 

260 

22. 
23. 

24. 

Dull  red  heat     .... 
Melting  point  of  alumin- 
ium      
Melting  point  of   com- 

650 
660 

4. 

Boiling  point  of  hydrogen 

252.6 

25 

mon  salt      .... 
Red  heat  

792 
800 

5. 

Melting  point  of  nitrogen 

214 

26. 

97 

Bright  red  heat     .     .     . 

1000 

1  nfi4. 

6. 

Boiling  point  of  nitrogen 

194 

28. 
29. 

Melting  point  of  copper 
Yellow  heat  

1090 
1200 

7. 

Boiling  point   of    oxygen 

30 

White  heat   

1350 

_ 

182.5 

8. 

Freezing  point  of  grain  al- 
cohol   ....          — 

130 

31. 

Temperature    of     glass 
furnace   .     .     .     .     ; 

1375 

9. 

Freezing  point  of  mercury 

-  39  5 

32. 
33. 

Melting  point  of  iron     . 
Melting  point  of   plati- 

1520 

num    

1750 

10. 
11. 

Freezing  point  of  water 
Average    room    tempera- 

0 

34. 

Melting  point  of  corun- 
dum     

2000  ± 

ture    

20 

12. 
13. 
14. 

Normal    temperature    of 
the  body     

Melting  point  of  Wood's 
metal      

Boiling  point  of  grain  al- 
cohol     .... 

37 
60 
785 

35. 
36. 
37. 

00 

Melting  point  of    irid- 
ium     
Temperature  of  oxyhy- 
drogen  flame   .     .     . 
Melting    point    of    os- 
mium   

2300 
2500  ± 
2500 

15. 

Boiling  point  of  water  .     . 

100 

acetylene  flame     .     . 
above 

2700 

16. 

Melting  point  of  rhombic 
sulphur  . 

114  5 

39. 

Melting  point  of  tung- 

17. 
18. 
19. 
20. 

Melting  point  of  tin     .     . 
Melting  point  of  lead   .     . 
Melting  point  of  zinc   .     . 
Boiling  point  of  sulphur  . 

232 
327 
419 
445 

40. 
41. 

sten     
Temperature     of      alu- 
mino-thermy   .     .     . 
Temperature  of  electric 
arc 

3000 
3500  ± 
4000  ± 

21. 

Incipient  red  heat    .     .     . 

550 

42. 

Temperature  of  sun  .     . 

6000 

B.    AND   W.    CHEM.  28 


432 


APPENDIX 


IX.     COMPOSITION  OF  ALLOYS 


TYPE  OP  ALLOY 

NAME 

APPROXIMATE  COMPOSITION 
IN  PARTS  PER  100 

Copper  Alloys 

Brass 

Cu  (67-80)  ;  Zn  (20-33)  ;  Pb  \± 

Bronze 

Cu  84;  Pb  1±;  Sn  5;  Zn  10 

Aluminium  bronze 

Cu  (90-95)  ;  Al  (5-10) 

Manganese  bronze 

Cu  70  ;  Mn  30 

Gun  metal 

Cu  90  ;  Sn  10 

Bell  metal 

Cu  75  ;  Sn  25 

Dutch  metal 

Cu  80  ;  Zn  20 

Copper  coins 

Cu  95  ;  Sn  4  ;  Zn  1 

5-cent  piece 

Cu  75  ;  Ni  25 

German  silver 

Cu  (55-60)  ;  Zn  20  ;  Ni  (20-25) 

Iron  Alloys 

Steel 

Fe  (99-99.75);  C  (0.25-1) 

Nickel  steel 

Fe94;  C  1±  ;  Ni  5± 

Chrome  steel 

Fe  (94^-96*)  ,  C  (1-1*)  ; 

Cr  (2J-4±) 

Tungsten  steel 

Fe94;  C  1±;  W  5± 

Manganese  steel 

Fe91;  C  1±;  Mn  8± 

Molybdenum  steel 

Fe  (89-94)  ;  C  1±  ;  Mo  (5-10) 

Lead-Tin  Alloys 

Solder 

Pb  (33-66)  ;  Sn  (33-66) 

Pewter 

Sn  75  ;  Pb  25 

Shot  metal 

Pb99;  As  1± 

Type  metal 

Pb  75  ;  Sb  20  ;  Sn  5 

Britannia  metal 

Sn  85  ;  Sb  10  ;  Cu  5 

Silver  Alloys 

Sterling  silver 

Ag92.5;  Cu7.5 

Coin  silver 

Ag  90  ;  Cu  10 

Gold  Alloys 

U.S.  gold  coinage  21.  6k 

Au  90  ;  Cu  10 

18  k  gold 

Au  75  ;  Cu  25 

14  k  gold 

Au58.3;  Cu  41.7 

10  k  gold 

Au41.6;  Cu  58.4 

Fusible  Alloys 

Wood's  metal 

Bi  50  ;  Pb  25  ;  Cd  12.5  ;  Sn  12.5 

(melts  at  60.5°  C.) 

Rose's  metal 

Bi  82  ;  Pb  9  ;  Sn  9 

(melts  at  94°  C.) 

Miscellaneous 

Babbitt's  antifriction 

Alloys 

metal 

Zn  69  ;  Sn  19  ;  Pb  5  ;  As  4  ;  Sb  3 

APPENDIX 


433 


X.    COMPOSITION    OF    TERRESTRIAL    MATTER    TO    A 
DEPTH   OF   10  MILES 


SOLID  CRUST 
OF  EARTH, 

93    PER   CENT 

OCEANS, 

7    PER    CENT 

AVERAGE, 
INCLUDING 
ATMOSPHERE 

Oxygen  
Silicon  .'  
Aluminium  
Iron  .  

47.17 
28.00 

7.84 
4.44 

85.79 

49.85 
26.03 
7.28 
4.12 

Calcium  .  ...  . 

3.42 

.05 

3.18 

Magnesium 

2  27 

.14 

2.11 

Sodium  
Potassium  
Hydrogen  
Titanium 

2.43 
2.49 
.23 
44 

1.14 
.04 
10.67 

2.33 
2.33 
.97 
41 

19 

002 

19 

Chlorine  

.06 

2.07 
008 

.20 

Phosphorus  
Sulphur  
Barium  
Manganese  
Strontium  
Nitrogen 

.11 
.11 
.09 
.08 
.03 

'  .09  ' 

.10 
.10 
.09 
.08 
.03 
03 

Fluorine 

10 

10 

All  other  elements  

.50 

.47 

100.00 

100.00 

100.00 

The  average  density  of  the  solid  crust  to  a  10-mile  depth  is  2.6.  The 
mean  density  of  the  ocean  is  1.027.  The  average  density  of  the  whole  earth 
is  somewhat  over  5.  The  average  content  of  dissolved  salts  in  ocean  water 
is  3.5  per  cent. 

COMPOSITION  OF   OCEANIC  SALTS 

Sodium  chloride,  NaCl 77.76 

Magnesium  chloride,  MgCU 10.88 

Magnesium  sulphate,  MgSCh 4.74 

Calcium  sulphate,  CaSC>4 ..." 3.60 

Potassium  sulphate,  K2SO4 2.46 

Magnesium  bromide,  MgBr2 22 

Calcium  carbonate,  CaCOs .34 

100.00 


The  above  figures  were  calculated  by  F.  W.  Clarke  and  published  in  1911 
in  Bulletin  491  of  The  United  States  Geological  Survey  entitled  The  Data  of 
Geochemistry. 


XL  — Logarithms 


Nat. 
Number 

0 

1 

2 

3 

4 

5 

6 

'7 

8 

9 

PROPORTIONAL 

PARTS 

10 
11 
12 
13 
14 

15 
16 
17 
18 
19 

0000 
0414 
0792 
1139 
1461 

1761 
2041 
2304 
2553 

2788 

0043 
0453 
0828 
1173 
1492 

1790 
2068 
2330 
2577 
2810 

0086 
0492 
0864 
1206 
1523 

1818 
2095 
2355 
2601 
2833 

0128 
0531 
0899 
1239 
1553 

1847 
2122 
2380 
2625 
2856 

0170 
0569 
0934 
1271 
1584 

1875 
2148 
2405 
2648 
2878 

0212 
0607 
0969 
1303 
1614 

1903 
2175 
2430 
2672 
2900 

0253 
0645 
1004 
1335 
1644 

1931 
2201 
2455 
2695 
2923 

0294 
0682 
1038 
1367 
1673 

1959 
2227 
2480 
2718 
2945 

0334  0374 
07190755 
1072  1106 
1399  1430 
1703  1732 

1987  2014 
2253  2279 
2504  2529 
2742  2765 
2967  2989 

4    8  12;  17  21  25  29  33  37 
4    8  ll!  15  19  23  26  30  34 
3    710  141721  242831 
3    610131619232629 
369  121518212427 

3    6    8  111417  202225 
3    5    8  111316  182124 
2    5    7  101215  172022 
2    5    7    91214  161921 
247    91113161820 

20 
21 
22 
23 
24 

25 
26 
27 

28 
29 

3010 
3222 
3424 
3617 
3802 

3979 
4150 
4314 
4472 
4624 

3032 
3243 
3444 
3636 
3820 

3997 
4166 
4330 
4487 
4639 

4786 
4928 
5065 
5198 
5328 

5453 
5575 
5694 
5809 
5922 

3054 
3263 
3464 
3655 
3838 

4014 
4183 
4346 
4502 
4654 

4800 
4942 
5079 
5211 
5340 

5465 
5587 
5705 
5821 
5933 

6042 
6149 
6253 
6355 
6454 

6551 
6646 
6739 
6830 
6920 

3075 
3284 
3483 
3674 
3856 

4031 
4200 
4362 
4518 
4669 

4814 
4955 
5092 
5224 
5353 

5478 
5599 
5717 
5832 
5944 

3096 
3304 
3502 
3692 
3874 

4048 
4216 
4378 
4533 
4683 

4829 
4969 
5105 
5237 
5366 

5490 
5611 
5729 
5843 
5955 

3118 
3324 
3522 
3711 
3892 

4065 
4232 
4393 
4548 
4698 

3139 
3345 
3541 
3729 
3909 

4082 
4249 
4409 
4564 
4713 

4857 
4997 
5132 
5263 
5391 

5514 
5635 
5752 

5866 
5977 

3160 
3365 
3560 
3747 
3927 

4099 
4265 
4425 
4579 

4728 

4871 
5011 
5145 
5276 
5403 

5527 
5647 
5763 

5877 
5988 

3181 
3385 
3579 
3766 
3945 

4116 
4281 
4440 
4594 
4742 

4886 
5024 
5159 
5289 
5416 

5539 
5658 
5775 

5888 
5999 

3201 
3404 
3598 
3784 
3962 

4133 
4298 
4456 
4609 
4757 

246 
246 
246 
246 
245 

235 
235 
235 
235 
1    3    4 

81113 
81012 

81012 
7    911 
7    911 

7    910 
7    810 
689 
689 
679 

151719 
14  16  18 
141517 
131517 
121416 

121415 
111315 
111314 
111214 
10  12  13 

30 
31 
32 
33 
34 

35 
36 
37 
38 
39 

4771 
4914 
5051 
5185 
5315 

5441 
5563 
5682 
5798 
5911 

4843 
4983 
5119 
5250 
5378 

5502 
5623 
5740 
5855 
5966 

4900 
5038 
5172 
5302 

5428 

5551 
5670 

5786 
5899 
6010 

134 
1    3    4 
1    3    4 
1    3    4 
134 

1    2    4 
124 
1    2    3 
1    2    3 
1    2    3 

679 
678 
578 
568 
568 

567 
567 
567 
567 
457 

101113 
1011  12 
911  12 
91012 
91011 

91011 
81011 
8    910 
8    910 
8    910 

40 
41 
42 
43 
44 

45 
46 
47 

48 
49 

6021 
6128 
6232 
6335 
6435 

6532 
6628 
6721 
6812 
6902 

6031 
6138 
6243 
6345 
6444 

6542 
6637 
6730 
6821 
6911 

6053 
6160 
6263 
6365 
6464 

6561 
6656 
6749 
6839 
6928 

7016 
7101 
7185 
7267 
7348 

6064 
6170 
6274 
6375 
6474 

6571 
6665 
6758 
6848 
6937 

7024 
7110 
7193 
7275 
7356 

6075 
6180 
6284 
6385 
6484 

6580 
6675 
6767 
6857 
6946 

7033 

7118 
7202 
7284 
7364 

6085 
6191 
6294 
6395 
6493 

6590 
6684 
6776 
6866 
6955 

6096 
6201 
6304 
6405 
6503 

6599 
6693 
6785 
6875 
6964 

6107 
6212 
6314 
6415 
6513 

6609 
6702 
6794 
6884 
6972 

6117 
6222 
6325 
6425 
6522 

6618 
6712 
6803 
6893 
6981 

1    2    3 
1    2    3 
1    2    3 
123 
1    2    3 

1    2    3 
1    2    3 
1    2    3 
123 
123 

456 
456 
456 
456 
456 

456 
456 
455 
445 
445 

8    910 

789 
789 
789 
789 

789 

778 
678 
678 
678 

50 
51 
52 
53 
54 

6990 
7076 
7160 
7243 
7324 

6998 
7084 
7168 
7251 
7332 

7007 
7093 
7177 
7259 
7340 

7042 
7126 
7210 
7292 
7372 

70507059 
71357143 
72187226 
7300  7308 
73807388 

7067 
7152 
7235 
7316 
7396 

1    2    3 
123 
122 
122 
1    2    2 

345 
345 
345 
345 
345 

678 
678 
677 
667 
667 

0 

| 

2 

3 

4 

5 

6 

7 

8 

9 

123 

456 

789 

PROPORTIONAL 

PARTS 

434 


XL —Logarithms 


Nat. 
Number 

0 

I 

2 

3 

7427 
7505 
7582 
7657 
7731 

7803 
7875 
7945 
8014 
8082 

4 

5 

7443 
7520 
7597 
7672 

7745 

7818 
7889 
7959 
8028 
8096 

8162 
8228 
8293 
8357 
8420 

8482 
8543 
8603 
8663 

8722 

6 

7 

8 

7466 
7543 
7619 
7694 
7767 

7839 
7910 
7980 
8048 
8116 

8182 
8248 
8312 
8376 
8439 

8500 
8561 
8621 
8681 
8739 

9 

PROPORTIONAL   PARTS 

1    2    34     5    6 

789 

55 
56 
57 

58 
59 

60 
61 
62 
63 
64 

7404 
7482 
7559 
7634 
7709 

7782 
7853 
7924 
7993 
8062 

7412  7419 
7490  7497 
7566  7574 
7642  7649 
7716  7723 

77897796 
7860:7868 
7931  7938 
80008007 
8069  8075 

7435 
7513 

7589 
7664 

7738 

7810 
7882 
7952 
8021 
8089 

7451 

7528 
7604 
7679 
7752 

7825 
7896 
7966 
8035 
8102 

8169 
8235 
8299 
8363 
8426 

8488 
8549 
8609 
8669 

8727 

7459 
7536 
7612 
7686 
7760 

7832 
7903 
7973 
8041 
8109 

8176 
8241 
8306 
8370 
8432 

8494 
8555 
8615 
8675 
8733 

7474 
7551 
7627 
7701 
7774 

7846 
7917 
7987 
8055 
8122 

2    2 
2    2 
2    2 
1     2 
1    2 

1    2 
1    2 
1    2 
1    2 
1    2 

345 

345 
345 
344 
344 

344 
344 
334 
334 
334 

6    7 
6    7 
6    7 
6    7 
6    7 

6    6 
6    6 
566 
556 
556 

65 
66 
67 
68 
69 

70 
71 
73 
73 
74 

8129 
8195 
8261 
8325 
8388 

8451 
8513 
8573 
8633 
8692 

8136 
8202 
8267 
8331 
8395 

8457 
8519 
8579 
8639 
8698 

81428149 
82098215 
8274  8280 
8338  8344 
8401  8407 

8463  8470 
85258531 
8585  8591 
8645  8651 
8704  8710 

8156 
8222 
8287 
8351 
8414 

8476 
8537 
8597 
8657 
8716 

8189 
8254 
8319 
8382 
8445 

8506 
8567 
8627 
8686 
8745 

1     1    2 
112 
1          2 
1          2 
1          2 

1          2 
1          2 
1          2 
1          2 
1          2 

334 
334 
334 
334 
234 

234 
234 
234 
234 
234 

556 
556 
556 
456 
456 

456 
455 
455 
455 
455 

75 

76 
77 

78 
79 

80 

81 
83 
83 

84 

8751 
8808 
8865 
8921 
8976 

9031 
9085 
9138 
9191 
9243 

8756  8762  8768 
8814:88208825 
8871  8876  8882 
8927  89328938 
898289878993 

9036  9042  9047 
909090969101 
9143'9149  9154 
9196  9201  9206 
9248^253  9258 

8774 
8831 
8887 
8943 
8998 

9053 
9106 
9159 
9212 
9263 

8779 
8837 
8893 
8949 
9004 

9058 
9112 
9165 
9217 
9269 

8785 
8842 
8899 
8954 
9009 

9063 
9117 
9170 
9222 
9274 

8791 
8848 
8904 
8960 
9015 

9069 
9122 
9175 
9227 
9279 

8797 
8854 
8910 
8965 
9020 

9074 
9128 
9180 
9232 
9284 

8802 
8859 
8915 
8971 
9025 

9079 
9133 
9186 
9238 
9289 

1          2 
1          2 
1          2 
1          2 
1          2 

1          2 
1          2 
1          2 
1          2 
1          2 

233 
233 
233 
233 
233 

233 
233 
233 
233 
233 

455 
455 
445 
445 
445 

445 
445 
445 
445 
445 

85 
86 
87 
88 
89 

90 
91 
93 
93 
94 

9294 
9345 
9395 
9445 
9494 

9542 
9590 
9638 
9685 
9731 

9299 
9350 
9400 
9450 
9499 

9547 
9595 
9643 
9689 
9736 

9304 
9355 
9405 
9455 
9504 

9552 
9600 
9647 
9694 
9741 

9309 
9360 
9410 
9460 
9509 

9557 
9605 
9652 
9699 
9745 

9315!9320 
93659370 
9415  9420 
9465  9469 
95139518 

95629566 
9609  9614 
9657  9661 
9703  9708 
9750  9754 

9325 
9375 
9425 
9474 
9523 

9571 
9619 
9666 
9713 
9759 

9805 
9850 
9894 
9939 
9983 

9330 
9380 
9430 
9479 
9528 

9576 
9624 
9671 
9717 
9763 

9335 
9385 
9435 
9484 
9533 

9581 
9628 
9675 
9722 
9768 

9340 
9390 
9440 
9489 
9538 

9586 
8633 
9680 
9727 
9773 

1          2 
1          2 
0          1 
0          1 
0          1 

0          1 
0          1 
0          1 
0          1 
0          1 

233 

233 
223 
223 
223 

223 
223 
223 
223 
223 

445 

445 
344 
344 
344 

344 
344 
344 
344 
344 

95 
96 
97 
98 
99 

9777 
9823 
9868 
9912 
9956 

9782 
9827 
9872 
9917 
9961 

9786 
9832 
9877 
9921 
9965 

9791 
9836 
9881 
9926 
9969 

9795 
9841 
9886 
9930 
9974 

9800 
9845 
9890 
9934 
9978 

9809 
9854 
9899 
9943 
9987 

9814 
9859 
9903 
9948 
9991 

9818 
9863 
9908 
9952 
9996 

0          1 
0          1 
0          1 
0          1 
0          1 

223 

223 
223 
223 
223 

344 
344 
344 
344 
334 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

123 

456 

789 

PROPORTIONAL   PARTS 

435 


436  APPENDIX 


XII.   METRIC  SYSTEM 

The  standard  unit  of  the  Metric  System  is  the  meter. 
It  is  a  unit  of  length.  Its  multiples  and  submultiples  are 
obtained  decimally 

10  millimeters  (mm.)   =  1  centimeter  (cm.) 
10  cm.  =  1  decimeter  (dm.) 

10  dm.  =  1  meter  (m.) 

The  unit  of  volume  in  the  metric  system  is  the  liter.  It 
is  the  cubic  decimeter.  Like  the  meter  it  is  divided  decimally. 

1000  cu.  cm.  (c.c.)l 

,  •  .  \  =  1  liter 

or  1  cu.  decimeter  J 

The  unit  of  weight  is  the  gram.  It  is  the  weight  of  1  c.c. 
of  water  at  4°  C.  It  is  divided  decimally. 

10  milligrams  (mg.)  =  1  centigram  (eg.) 

10  eg.  =  1  decigram  (dg.) 

10  dg.  =  1  gram  (g.) 

1000  g.  =  1  kilogram  (kg.) 

XIII.  TABLE  OF  EQUIVALENTS 

Showing    relation    between    some    frequently    used    metric 
measures  and  their  English  equivalents. 

1  meter  =  39.37  inches 
1  centimeter  =      .3937  inch 

1  inch  =    2.54  centimeters 

1  liter  =    1.0567  quarts 
1  cubic  centimeter  =      .061  cubic  inch 

1  gram         =  15.43  grains 
1  kilogram  =    2.2046  Ib. 
1  ounce        =  28.35  grams 


INDEX 


Absolute  temperature,  81. 
Absolute  zero,  80. 
Acetic  acid,  293. 
Acetone,  from  wood,  265. 
Acetylene,  276,  279,  287. 
Acid,  acetic,  293. 

anhydrides,  211. 

carbonic,  208. 

forming  elements,  210. 

hydrobromic,  382. 

hydrochloric,  132. 

hydrogen,  211. 

hypochlorous,  171. 

nitric,  366. 

muriatic,  132. 

phosphoric,  209. 

radicals,  216. 

sulphuric,  209. 
\      sulphurous,  208. 

Acids,  characteristic  component,  207. 

fatty,  294,  295. 

nomenclature,  220-223. 

organic,  293. 

properties  of,  207. 
Air,  composition,  67,  72. 

determination  of  oxygen  in,  74-77. 

liquid,  40. 

mixture,  69. 

questions  on,  78. 

summary,  77. 
Alabaster,  201. 
Alcohol,  291. 

denatured,  292. 

wood,  293. 

Alkali  metals,  230.     - 
Alkaline,  212. 
Alkaline  earth  metals,  233. 
Alloys,  229. 

brass,  229,  255. 

bronze,  229. 

composition  of,  Table,  432. 


Alloys,  solder,  229,  257. 

type  metal,  257. 
Aluminium,  232-234. 

metallurgy,  260. 

ore,  240,  261. 

uses,  261. 

Aluminothermy,  415. 
Aluminum,  see  Aluminium. 
Amalgamation  process,  gold,  259. 
Ammonia,  358. 

commercial,  359. 

preparation,  360. 

properties,  360. 

refrigeration,  360. 

water,  217,  360. 
Ammonium  carbonate,  362. 

chloride,  362. 

hydroxide,  217. 

nitrate,  362. 

radical,  217,  232. 

salts,  362. 
Amorphous  carbon,  265. 

sulphur,  344. 
Anhydride,  acid,  211. 

basic,  214. 
Appendix,  425-436. 
Aqua  regia,  170,  236. 
Aqueous  pressure,  425. 
Argon,  in  air,  72. 
Artificial  diamond,  269. 
Artificial  fertilizers,  372. 
Artificial  gems,  117. 
Artificial  graphite,  267. 
Artificial  silk,  371. 
Atmosphere,  67. 

Atomic      and      molecular      weights 
(Chap.  XV),  147. 

questions,  163. 

summary,  162. 
Atomic  theory  (Chap.  XII),  121. 

questions,  128. 


437 


438 


INDEX 


Atomic  theory,  summary,  127. 

value,  126. 
Atomic  weights,  determination,  149. 

practical  use,  157. 

standard,  148. 

Table,  common  elements,  340. 

Table,  International.     Inside  back 

cover. 
Atoms,  absolute  weight,  147. 

possible  complexity,  127. 

relations  to  electrons,  314. 

relative  weight.  147. 
Avogadro's  Theory  (Chapter   XIV), 
141. 

applications,  142. 

scope,  144. 

summary,  146. 

Bacteria,  and  decay,  47. 

cause  oxidation,  294. 

chemical  destruction,  97. 

in  drinking  water,  95. 

nitrogen  fixing,  371. 
Baking  powder,  183. 
Baking  soda,  180. 
Balloons,  hydrogen,  113. 

illuminating  gas,  119. 

lifting  power  of,  113. 
Barium,  232,  233. 
Barometer,  76. 
Base,  211,  212. 
Bases,  214. 

characteristic  component,  214. 

nomenclature,  223. 
Basic  oxides,  214. 
Battery,  the  storage,  423. 
Bauxite,  234,  261. 
Benzine,  287. 
Bessemer  steel,  245. 
Bicarbonate  of  soda,  180. 
Blast  furnace,  241. 

diagram,  242. 
Bleaching,  chlorine,  172. 

powder,  172-174. 

theory,  172. 

Blowpipe,  oxy hydrogen,  116. 
Borax,  406. 
Boyle's  law,  84. 

explained,  145. 

Bread  and  cake,  raising,  61,  62. 
Bromides,  382. 
Bromine,  380. 


Burning,  see  Combustion. 
Butane,  285. 

Calcium,  192,  232,  233. 

bicarbonate,  199. 

carbide,  278,  421. 

carbonate,  195. 

chloride,  193. 

drying  gases,  194. 

fluoride,  379. 

hydroxide,  197,  213. 

oxide,  196,  213. 

questions,  205. 

sulphate,  201. 

summary,  204. 
Calculations,  chemical,  157. 
Candle,  products  of  combustion,  28. 
Cane  sugar,  289. 
Carbides,  calcium,  278. 

iron,  276. 

silicon,  277. 
Carbohydrates,  288. 
Carbon,  amorphous,  265. 

crystalline,  267. 

diamond,  268. 

oxides,  31,  50. 
Carbonates,  270. 
Carbonate,  calcium,  195. 
Carbonate,  sodium,  180-185. 
Carbon  cycle,  419. 
Carbon  dioxide,  50. 

chemical  properties,  58. 

commercial  production,  62. 

enormous  production,  50. 

from  burning  charcoal,  11. 

from  limestone,  51. 

generator,  53. 

heat  of  formation,  272. 

in  air,  72. 

liquid,  56. 

physical  properties,  55. 

properties,  53-58. 

questions,  65. 

solid,  57. 

summary,  64. 

test,  52. 

uses,  58-61. 

utilization  by  plants,  50. 
Carbon  disulphide,  279. 
Carbon  monoxide,  62,  272. 

blast  furnace,  244. 

combining  proportions,  63. 


INDEX 


439 


Carbon  monoxide,  formation,  62. 

poisonous,  270. 

properties,  63. 

uses,  63. 

Carbon  tetrachloride,  279. 
Carborundum,  277. 

furnace,  278. 
Cast  iron,  245. 
Catalysis,  40. 
Caustic,  230. 
Caustic  soda,  185. 
Celluloid,  371. 
Cellulose,  290. 

nitrate,  370. 
Cement,  201. 
Cerium,  276. 

Chamber  process,  sulphuric  acid,  352. 
Charcoal,  265. 

reducing  agent,  241. 
Charles'  law,  80,  82,  83. 

example  of  calculation,  83. 

explained,  146. 

modern  expression,  83. 
Chemical  affinity,  398. 
Chemical     and     physical     changes, 

questions,  14. 

summary,  14. 
Chemical  attraction,  228. 
Chemical  change,  9. 

a  typical,  10. 

definition,  12. 

examples,  12. 
Chemical  compounds,  21. 
Chemical  energy,  413. 
Chemical  formulas,  155. 
Chemical  substances,  15. 
Chemistry,  definition,  9. 

organic,  282. 
Chili  saltpeter,  366,  372. 

origin,  373. 
Chloride  of  lime,  174. 
Chlorine,  action  on  metals,  168. 

and  carbon,  169. 

and  hydrogen,  168. 

and  oxygen,  174. 

bleaching,  173. 

chemical  properties,  168. 

commercial  preparation,  166. 

laboratory  preparation,  167. 

liquid,  174. 

physical  properties,  167. 

questions,  176. 


Chlorine,  source  of,  164. 

summary,  175. 

water,  170. 
Chloroform,  284. 
Chlorophyll,  289,  372. 
Chromium,  234. 
Cleaning,  230. 
Coal,  266. 

hard,  266. 

soft,  266. 
Coal  gas,  273. 
Coal  tar,  273. 
Coke,  265. 

from  petroleum,  287. 

in  blast  furnace,  241,  242. 
Collodion,  371. 
Combustion,  historic  views,  25. 

in  pure  oxygen,  44. 

Lavoisier's  experiment,  30. 

modern  theory,  26,  30. 

questions,  36. 

slow,  46,  264. 

spontaneous,  46. 

summary,  36. 
Common  salt,  179. 
Compound,  formation  of  a   typical, 

23. 
Compounds,  chemical,  21. 

questions,  24. 

summary,  24. 

Conduction,    electrolytic,    302,    303, 
304. 

metallic,  301. 

Conservation  of  matter,  law  of,  11. 
Contact  process,  sulphuric  acid,  351. 
Converter,  Bessemer,  246. 
Cooking  soda,  see  Baking  soda. 
Copper,  234,  235,  236,  237,  252. 

native,  252. 

ores,  240,  252. 

oxide,  236,  237,  240. 

plating,  321. 

refining,  253. 

smelting,  253. 

sulphate,  320,  321. 

sulphide,  236,  237,  252. 

uses,  255. 

Cream  of  tartar,  183. 
Cryolite,  260. 
Crystal  hydrates,  185. 
Cyanide  process,  gold,  259. 
Cymogene,  286. 


440 


INDEX 


Deacon's  process  for  chlorine,  165. 
Decay,  of  wood,  31,  47. 
Definite  proportions,  law,  22. 

illustrated,  106. 

stated,  121. 
Deliquescence,  194. 
Dextrin,  290. 
Diamond,  268. 

artificial,  269. 

uses,  269. 

Displacement,  of  hydrogen  by  metals 
328. 

of  iodine  by  other  halogens,  384. 

of  one  metal  by  another,  328. 
Distillation,  17. 

for  soft  water,  203. 

of  alcohol,  291. 

of  coal,  273. 

of  petroleum,  286. 

of  wood,  293. 
Draft,  forced,  33. 
Drinking  water,  95. 
Dynamite,  371,  418. 

Earth,  composition,  453. 

Earth  forming  metals,  230,  232. 

Effervescence,  52,  305. 

Efflorescence,  185. 

Electricity,    atomic     structure,    314, 

322. 

Electrochemical  equivalents,  331. 
Electrolysis  (Chap.  XXVI),  311. 

copper  sulphate,  320. 

hydrochloric  acid,  311. 

questions,  326. 

sodium  chloride,  320. 

sodium  hydroxide,  318. 

sodium  sulphate,  319. 

sulphuric  acid,  315. 

summary,  325. 

water,  103. 
Electrolytic  conduction,  302,  305. 

character,  303. 

dissociation,  304. 

Electromotive  series  (Chap.  XXVII), 
327. 

questions,  332. 

summary,  331. 
Electrons,  314. 

Electroplating  of  copper,  321. 
Electrostatic,   attraction    and   repul- 
sion, 299r 


Elements,  20. 

of  the  Greek  philosophers,  25. 

periodic  classification,  385. 

periodic  table,  389. 

possible  complexity,  21. 

proportion  in  earth's  crust,  433. 

questions,  24. 

summary,  24. 

table  of  properties,  427.       , 
Endothermic  reactions,  417. 
Endothermic     substances,     artificial, 

420. 
Energy     changes     and     the     carbon 

cycle,  419. 

Energy,  chemical,  413. 
Energy,  different  forms,  413. 
Equations,  156. 

example,  156. 

ionic,  304, 
Equilibrium  (Chap.  XXXII),  392. 

questions,  412. 

summary,  410. 

chemical,  395. 
Equivalents,  electrochemical,  331. 

hydrogen,  333. 

metric,  tables,  436. 
Ethane,  285,  288. 
Ethylene,  287,  288. 
Exothermic  compounds,  417. 
Exothermic  familiar  reactions,  416. 
Exothermic  reactions,  414. 
Explosions,  theory  of,  422. 

Faraday's  law,  322. 
Fats,  294. 

as  foods,  295. 
Fermentation,  acetic,  293-294. 

alcoholic,  291. 

Fertilizer,  72,  187,  372,  379. 
Filtration,  to  purify  water,  96. 
Fire,  25. 

extinguishers,  58,  59,  60,  280. 
Flame,  color,  187. 

luminosity,  275. 

oxyhydrogen,  116,  117. 
Fluorine,  378. 
Flux,  in  blast  furnace,  242. 
Food,  canning,  48. 

nitrogenous,  70,  372. 

spoiling,  47. 

Foods,  slow  combustion  in  body,  32. 
Formula,  how  to  find,  152. 


INDEX 


441 


Formulas,     condensed     information, 
154. 

of  some  compounds,  429. 

interpretation  of,  155. 
Formula  weights,  157. 
Fuels,  gaseous,  118. 
Furnace,  blast,  241-244. 

open  hearth,  247. 

reverberatory,  250. 

Galena,  240,  255. 
Galvanized  iron,  249,  258. 
Gas,  acetylene,  276,  279,  287. 

artificial,      118,      271,      273,      275, 
276. 

carbon,  266. 

coal,  273. 

density,  428. 

detonating,  115. 

illuminating,  271,  275,  276. 

producer,  271. 

volumes,  and  water  vapor,  426. 

water,  270. 

Gases,  separation  of  molecules,  145. 
Gas  laws  (Chap.  VIII),  79. 

questions,  89-90. 

summary,  88-89. 
Gasoline,  287. 
Gay     Lussac's     law     of     combining 

volumes,  141. 
Gems,  artificial  rubies,  117. 

artificial  sapphires,  117. 

diamond,  268. 
Generator,  carbon  dioxide,  53. 

hydrogen,  112. 

hydrogen  chloride,  129. 

producer  gas,  272. 

water  gas,  271. 
Glass,  189. 
Glucose,  289. 
Gluten,  296. 
Glycerine,  see  Glycerol. 
Glycerol,  295. 
Gold,  inactivity,  236. 

in  aqua  regia,  236. 

metallurgy,  259. 

occurrence,  237,  259. 

ore,  240,  259. 

resistance  to  corrosion,  236. 
Gram-atomic  weights,  337. 
Gram-molecular  volumo,  151. 
Gram-molecular  weights,  150. 


Grape  sugar,  289. 
Graphite,  266. 

artificial,  267. 
Guncotton,  370. 
Gunpowder,  370. 

smokeless,  371. 
Gypsum,  192,  201. 

Halogen  family  (Chap.  XXXI),  377. 

questions,  391. 

summary,  390. 
Hardness,  permanent,  of  water,  203. 

temporary,  of  water,  203. 
Hard  water,  201. 

softening,  201. 
Heat,  and  chemical  reaction,  413-422. 

nature  of,  145. 
Helium,  73. 
Hematite,  240. 
Hofmann,  apparatus,  104. 
Humidity,  93. 

and  human  comfort,  95. 

relative,  94. 
Hydriodic  acid,  384. 
Hydrobromic  acid,  382. 
Hydrocarbons,  284. 

unsaturated,  287. 
Hydrochloric  acid,  132. 

uses,  133. 

see  Hydrogen  chloride. 

manufacture,  133. 
Hydrofluoric  acid,  379. 

use,  380. 
Hydrogen  (Chapter  XI),  111. 

acid,  or  displaceable,  211,  299. 

a  measure  of  valence,  204,  336. 

as  a  metal,  227. 

burns  in  chlorine,  118. 

discovery,  111. 

displacement  by  metals,  327. 

equivalents,  333. 

from  acjds,  112. 

generator,  112. 

in  water,  100,  101,  111. 

liquefaction,  115. 

properties,  113. 

rapid  diffusion  of,  114. 

reducing  action  of,  118. 

summary  and  questions,  119. 

two  parts  in  water,  103. 
Hydrogen  chloride   (Chapter  XIII), 
129. 


442 


INDEX 


Hydrogen  chloride,  composition,  135 

formula,  139. 

great  solubility,  131. 

questions,  140. 

synthesis  of,  137. 

summary,  139. 

volume  composition,  136. 

weight  composition,  139. 
Hydrogen         equivalents         (Chap. 
XXVIII),  333. 

questions,  341. 

summary  341. 
Hydrogen  peroxide,  108,  422. 

preparation,  187,  422. 

uses,  187. 
Hydrogen  sulphide,  347. 

as  a  precipitant,  348. 

preparation,  347. 
Hydrolysis,  232,  405. 
Hydrolyzable  salts,  uses  of,  408. 
Hydrosulphuric  acid,  348. 
Hydroxide,  ammonium,  217. 

calcium,  213. 

sodium,  211. 
Hydroxides,  223,  232. 
Hydroxyl,  223. 
Hydroxyl  ion,  307. 
Hypochlorous  acid,  171. 

instability,  171. 
Hypothesis,  vs.  theory,  122. 

Avogadro's,  141. 

Iceland  spar,  195. 
Ice  making,  361. 
Illuminants,  271,  275,  276,  288. 
Illuminating  gas,  acetylene,  276. 

coal,  273. 

water,  270. 
Ingots,  copper,  252. 

iron,  249,  251. 
Inorganic  matter,  264. 
Iodine,  382. 

displacement,  384. 

uses,  383. 
Ionic  theory  (Chap.  XXV),  299. 

equations,  304. 

questions,  309. 

summary,  308. 

theory,  statement  of,  304. 

value,  306. 

lonization,  degree,  305. 
Ionized  substances,  reactivity,  305. 


Ions,  as  carriers,  312. 

discharge,  312. 

pairing,  304. 
Iron,  234,  235,  236,  237. 

carbide,  276. 

cast,  245. 
uses,  248. 

galvanized,  249. 

ingot,  251,  252. 

metallurgy,  240-245. 

ores,  240. 

wrought,  250. 

uses,  251. 
Irrigation,  93. 

Kerosene,  287. 
Kindling  point,  44. 

definition,  45. 
Krypton,  73. 

Lampblack,  265. 

Lavoisier  and  combustion,  30. 

Law,  121. 

Boyle,  84,  145. 

Charles,  82,  83,  146. 

conservation  of  matter,  11. 

definite   proportions,   22,   64,    107, 
121. 

Faraday,  322. 

Gay  Lussac,  141. 

mass,  398. 

multiple  proportions,  64,  125. 
Lead,  234,  236. 

ores,  240,  255. 

poisoning,  256. 

refining,  256. 

smelting,  255. 

uses,  256. 

Light  and  chemical  energy,  418. 
Light,  chemical  reaction,  137. 
Lime,  196. 

air  slaked,  199. 

calcium  oxide,  196. 

kiln,  196,  197. 

milk  of,  198. 

uses,  197-198. 
Lime,  slaked,  197,  199. 

uses,  198. 
Limestone,  195. 

caves,  200. 
Limewater,  198. 
Linde  apparatus,  41. 
Liquid  air,  41. 


INDEX 


443 


Liquors,  alcoholic,  292. 
Litmus,  with  acids,  211. 

with  bases,  212. 
Logarithms,  434,  435. 
Lubricating  oil,  287. 
Luminosity  of  flames,  275. 

Magnesium,  232,  233,  234. 
Mammoth  cave,  200,  201. 
Manganese,  heavy  metals,  234. 
Marble,  195. 
Marsh  gas,  284. 
Mass  law,  398. 
Matte,  copper,  252. 

lead,  256. 
Melting  points,  elements,  427,  431. 

compounds,  429,  431. 
Mercury,  234. 

metallurgy,  258. 

occurrence,  237. 

ore,  240,  258. 

oxide,  38. 

Metallic  conductor,  301. 
Metallic  radicals,  217. 
Metals  (Chapter  XXI),  225. 

alkali,  230. 

alkaline  earth,  233. 

as  base  formers,  211. 

chemical  properties,  226. 

classification,  230. 

compounds  with  other  metals,  229. 

earth-forming,  232. 

heavy,  234. 

oxidation  of,  27. 

physical  properties,  225. 

tarnishing,  236. 

varying  activity,  235. 
Methane,  284. 

Methyl  alcohol,  see  Wood  alcohol. 
Metric  system,  436. 

equivalents,  436. 
Milk  of  lime,  198. 
Minerals,  239. 

table  of  important,  240. 
Mixtures  (Chap.  II),  15. 

and  compounds,  22. 

methods  of  separating,  16. 

iron  and  sulphur,  22. 

questions  and  summary,  19. 
Moisture,  atmospheric,  93. 
Molal  volume,  151. 

finding  formulae,  152. 


Molecular  weights,  150. 

how  determined,  152. 

practical  use,  157. 
Molecules,  127. 

elementary  gases,  142,  153. 
Moles,  150,  152. 

Monoclinic  sulphur  crystals,  344. 
Mortar,  198. 

Multiple    proportions,    among   ionic 
charges,  321. 

law  of,  64. 

law  stated,  125. 
Muriatic  acid,  see  Hydrochloric  acid. 

Naphtha,  287. 

Natural  families  of  elements,  377. 

Neon,  73. 

Neutralization,  212. 

according  to  ionic  theory,  306. 

formation  of  water,  215. 

weak  acids  and  weak  bases,  401. 
Nickel,  the  heavy  metals,  234. 
Nitrate,  cellulose,  370. 

sodium,  187,  366,  372,  373. 
Nitric  acid,  366. 

properties,  366. 

reduction,  368. 
Nitric  oxide,  364,  420. 
Nitrides,  358. 

Nitrogen     and     compounds     (Chap. 
XXX),  358. 

questions,  375. 

summary,  374. 
Nitrogen,  artificial  fixation,  71. 

explosive  compounds,  370. 

fixation  of  by  bacteria,  371. 

fixation  of  by  legumes,  71. 

from  air,  67. 

inertness  of  free,  70. 

properties,  68,  69. 

pure,  preparation,  68. 

tetroxide,  365. 

Nitrogen  cycle  in  nature,  373. 
Nitroglycerin,  371,  420. 
Nitrous  oxide,  363. 
Nomenclature  (Chap.  XX),  220. 

bases,  223. 

binary  acids,  221. 

binary  compounds,  220. 

oxy-acids,  221. 

questions,  224. 

salts,  223. 


444 


INDEX 


Non-conductors,  electricity,  301. 
Non-electrolytes,  303. 
Non-ionized  substances,  305. 
Non-metals,  acid  formers,  210. 
compounds  with  non-metals,  229. 

Oil,  crude,  286. 

kerosene,  287. 

Oil  of  vitriol,  see  Sulphuric  acid. 
Oils,  vegetable,  295. 
Open  hearth  furnace,  247. 
Ore,  aluminium,  240,  260. 

antimony,  240. 

copper,  240,  252. 

gold,  240,  259. 

iron,  240,  241. 

lead,  240,  255. 

mercury,  240,  258. 

nickel,  240. 

platinum,  native,  240. 

silver,  240,  258. 

tin,  240. 

zinc,  240,  257. 
Ores,  239. 

Organic  chemistry,  282. 
Oxidation,  27,  31,  32,  44,  46,  47. 
Oxides,  naming,  44. 
Oxygen  (Chapter  V),  38. 

chemical  properties,  43,  44. 

discovery,  29. 

liquid  and  solid,  43. 

physical  properties,  40,  41,  42. 

preparation,  38,  39,  40. 

preparation  from  sodium  peroxide, 
187. 

questions,  48. 

unites  with  charcoal,  11. 

summary,  48. 

Oxyhydrogen  blowpipe,  116. 
Ozone,  159,  422. 

properties  of,  160. 

use  in  purifying  water,  161. 

Paraffin,  287. 

series,  285. 
Periodic  classification,  385. 

table,  389  and  facing  inside  back 

cover. 

Petroleum,  distillation,  286. 
Phlogiston,  26. 
Phosphoric  acid,  210. 
Phosphorus,  action  in  air,  67. 


determination  of  oxygen  in  air,  74. 

peritoxide,  209. 
Photosynthesis,  418. 
Physical  change,  examples,  13. 

questions  and  summary,  14. 
Plants,  and  carbon  cycle,  419. 

legumes,  71. 
Plaster,  198. 
Plaster  of  Paris,  201. 
Platinum,  236. 

occurrence,  237. 

resistance  to  corrosion,  236. 
Potassium,  188. 

natural  occurrence,  188. 

properties,  230. 
Potassium  peroxide,  231. 
Potassium  salts,  fertilizer,  189. 
Powder,  gun,  370. 

smokeless,  371. 
Precious  metals,  236. 
Precipitates,  399. 
Precipitation,  moisture,  94. 

with  hydrogen  sulphide,  348. 
Pressure,  standard,  86. 

water  vapor,  table,  425. 
Priestley,  discovery  of  oxygen,  29. 
Printer's  ink,  266. 

Problems  or  questions,  14,  19,  24,  36, 
48,  65,  78,  89,  98,  109,  119,  128, 
140,  163,  176,  191,  205,  218,  224, 
238,  262,  281,  298,  309,  326,  332, 
341,  356,  375,  391,  412,  424. 
Producer  gas,  271. 
Propane,  285. 
Protein,  296. 
Puddling,  250. 
Purification,  water,  96. 

Questions  and  problems,  14,  19,  24. 
36,  48,  65,  78,  89,  98,  109,  119, 
128,  140,  163,  176,  191,  205,  218, 
224,  238,  262,  281,  298,  309,  326, 
332,  341,  356,  375,  391,  412,  424, 

Quicklime,  196. 

Radicals,  acid,  216. 

metallic,  217. 
Ramsay,  73. 
Rare  gases,  in  air,  73. 
Rayleigh,  73. 
Reduction,  by  copper,  364. 

by  hydrogen,  118. 


INDEX 


445 


Reduction  of  iron  ore,  243. 

of  nitric  acid,  368. 
Refining,  copper,  253. 
Refrigeration,  360. 
Relative  humidity,  94. 
Reverberatory  furnace,  250. 
Reversible  reactions  (Chap.  XXXII), 
392. 

by  concentration,  394. 

by  heat,  392. 

equilibrium  point,  395,  396. 

questions,  412. 

summary,  410. 
Rhigolene,  286. 
Rochelle  salt,  183. 
Rubies,  artificial,  117. 
Rum,  292. 
Rust,  iron,  236. 

Salt,  common,  179. 

as  a  preservative,  179. 
Saltpeter,  Chili,  187,  372. 

source,  373. 

Salts,  from  neutralization,  215. 
Sapphire,  artificial,  117. 
Secondary    reactions    at    electrodes, 

317. 

Silicon,  carbide,  277. 
Silver,  234,  237. 

blackening  of,  236. 

metallurgy,  258. 

occurrence,  237. 

ores,  240,  258. 
Slag,  blast  furnace,  242. 
Slaked  lime,  197. 
Smelling  salts,  362. 
Smokeless  powder,  371. 
Smoke  preventer,  33.    • 
Soap  and  hard  water,  201. 
Soaps,  295,  406. 
Soda,  baking,  180. 

washing,  183. 
Soda  water,  60. 
Sodium  (Chap.  XVII),  177. 

chemical  properties,  178,  231. 

flame  test,  187. 

metallic,  177,  230. 

physical  properties,  178,  230. 

preparation,  177. 

questions,  191. 

summary,  190. 
Sodium  bicarbonate,  180. 


Sodium  bicarbonate,  Solvay  process, 
181. 

uses,  181. 
Sodium  carbonate,  183. 

uses,  183. 

Sodium  chloride,  179. 
Sodium  hydroxide,  185,  211. 

properties  and  uses,  186. 
Sodium  nitrate,  187,  366,  372,  373. 
Sodium  oxide,  211. 
Sodium  peroxide,  187,  211,  231. 
Sodium  stearate,  295. 
Solubilities,  table,  430. 

rules  of,  430. 
Solubility  and  temperature,  92. 

limit,  92. 

Solvay  process,  181. 
Soot,  266,  275. 
Slag,  blast  furnace,  243,  245. 

copper,  253. 
Slow  combustion,  264. 
Specific    gravity,    of    elements,    427, 
428. 

of  compounds,  429. 
Standard  conditions,  86. 

calculation,  86. 
Starch,  290. 

Stearic  acid,  295.  « 

Stearin,  294. 
Steel,  Bessemer,  245. 

low  carbon,  uses,  251. 

mild,  249. 

open  hearth,  247. 

special,  249. 
Sterilization,  97. 
Stoker,  chain  grate,  35. 
Storage  battery,  the,  423. 
Substance,  a  typical  chemical,  10. 
Substances,  chemical,  16. 

complexity,  18. 

questions  and  summary,  19. 
Sugars,  289. 

Sulphide,  hydrogen,  347. 
Sulphides,  346. 
Sulphur,  342. 

allotropic  forms,  343. 

occurrence,  345. 

uses,  342. 

Sulphur    and      Compounds     (Chap. 
XXIX),  342. 

questions,  356. 

summary,  355. 


446 


INDEX 


Sulphur  dioxide,  208,  349. 
Sulphur  oxides,  349. 
Sulphur  trioxide,  209,  350. 
Sulphuric  acid,  350. 

chamber  process,  352. 

contact  process,  351. 

properties,  354. 

Sunlight  in  photosynthesis,  418. 
Symbols,  chemical,  143. 

of  elements,  Table,  340  and  inside 
back  cover 

quantitative  significance,  154. 

Table,  atomic  weight  and  valence  of 
common  elements,  340. 

atomic    weight,    complete,     inside 
back  cover. 

chemical  symbols,   340  and  inside 
back  cover. 

composition  of  alloys,  432. 
earth's  crust,  433. 

density  of  gases,  428. 

electromotive  series,  329. 

important  temperatures,  431. 

logarithms,  434,  435. 

metric  and  equivalents,  436. 

oxides  of  nitrogen,  363. 

periodic     arrangement,     389     and 
facing  inside  back  cover. 

pressure    saturated    water    vapor, 
425. 

properties    of    common    elements, 
427,  428. 

properties  of  compounds,  429. 

solubilities  of  bases  and  salts,  430. 

sulphur  compounds,  356. 

valence  of  elements,  340. 
Tar,  from  coal,  273,  274. 

from  wood,  265. 
Temperature,  absolute,  81. 

rate  chemical  change,  45. 
Theory,  atomic,  122. 

Avogadro's,  141. 

electrolytic  dissociation,  304. 

ionic,  304. 
Thermit,  416. 
Thermometer,  absolute,  81. 

centigrade,  79,  80. 
Thorium,  Welsbach  mantle,  276. 
Tin,  234,  235. 
Tuyeres,  242. 


Valence,  203,  336. 

of  elements,  340. 

summary,  341. 

variable,  339. 
Vaseline,  287. 

Washing  soda,  183,  184. 
Water  (Chap.  IX),  91. 

a  solvent,  92. 

chemically  pure,  97. 

composition  (Chap.  X),  100. 

decomposition,  by  electricity,  103. 
by  hot  iron,  100.  . 

by  sodium,  102. 

drinking,  95. 

electrolysis,  103. 

gas,  270. 

hard,  201. 

industrial  purposes,  97. 

in  life  processes,  92. 

properties,  91. 

purification,  96. 

sterilization,  97. 

summary  and  questions,  98. 

summary  of  composition,  109. 

synthesis,  105. 
Water,  volume  composition,  103,  106. 

weight  composition,  107. 
Water  vapor,  in  air,  72. 

pressure  in  gases,  425. 
Weight  per  liter  of  gases,  428. 
Welsbach,  mantles,  276. 
Whiskey,  292. 
White  lead,  257. 
Whitewash,  198. 
Wood  alcohol,  293. 
Wood,  combustion  of,  31. 
Wrought  iron,  250,  251. 

Xenon,  73. 

Yeast,  action  on  fruit  sugar,  47. 
alcohol  production,  291. 
raising  bread,  61. 

Zero,  absolute,  80. 

centigrade,  80. 
Zinc,  257. 

battery  cells,  258. 

galvanizing,  258. 

ores,  240,  257. 


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INTERNATIONAL  ATOMIC  WEIGHTS   (1914) 
O=  16 


Uuminium     .     .     .     .    Al       27.1 
Antimony  Sb     120.2 

Molybdenum  .     . 
Neodymium    .... 
Neon  

Mo      96.0 
Nd    144.3 
Ne       20.2 
Ni       58.68 
Nt     222.4 
N        14.01 
Os      190.9 
O         16.00 
Pd     106.7 
P         31.04 
Pt      195.2 
K        39.10 
••*•  —  *40.6 
3.4 
2.9 
5.45 
1.7 
).4 
1.1 
).2 
5.3 
.88 
.00 
.03 
.07 
.5 
.5 
2 

0 
IY            4 

\rgon                       .     .    A        39.88 

\rsenic                              As       74.96 

Nickel 

Barium                    .     .    Ba    137.37 

Niton  (radium  emanation) 
Nitrogen     

Beryllium       ....    Be         9.1     ; 
Bismuth                   .         Bi     208.0 

Osmium  .    .         ... 

Boron    B         11.0 

jQxygen   

Bromine                       .    Br       79.92 

Palladium 

Cadmium       .    .     .    .    Cd    112.40 

Phosphorus     .... 
Platinum 

Csesium                             Cs     132.81 

Calcium     Ca      40.07 

Potassium  

Cerium       fc 
Chlorin  ! 

Chromi 

cobait                    302077 

Commit  * 
Copper  1 
Dyspr.oi 
Erbium  |# 
Europii 
Fluorin 
Gadolin  - 
Gallium 
German 
Gold 
Helium 
Holmiu 
Hydrog              UNIVERSITY  OF  CALIFORNIA  LIBRA* 

Iodine    I        126.92 
Iridium      Ir      193.1 
Iron            Fe       55.84' 

Tin     
Titanium         .... 

Sn      119.0 
Ti        48.1 
W      184.0 
U      238.5 
V         51.0 
Xe     130.2 

Yb     172.0 
Yt       89.0 
Zn       65.37 
Zr        90.6 

Tungsten     

Krypton                       .    Kr      82.92 

Uranium     

Lanthanum    ....    La     139.0 
Lead                                  Pb    207  10 

Vanadium  
Xenon     .                    . 

Lithium     Li          6.94 

Ytterbium   (Neoyttcr- 
bium)  

Lutecium                     •    Lu     174  0 

Magnesium    .     .     .     .     Mg     24.32 
Manganese  ..     .  ....    .     Mn     54.93 
Mercury    .     .  •  .     .-    .     Hg    200.6 

Yttrium  

Zinc    

Zirconium   .     . 

